Remove unused cc files [2/n]

Take one group from output of:
src/platform/libchrome/libchrome_tools/developer-tools/elftools/usagechecker.py
--libchrome-reference-board hatch --test-board hatch --print-groups

Shared object size -36K.

BUG=b:227987776
TEST=CQ

Change-Id: I2043e4a5aa57c12bfc4ad32d94ab5f43b6abe56e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/libchrome/+/3774853
Reviewed-by: Grace Cham <hscham@chromium.org>
Tested-by: Qijiang Yūki Ishii <fqj@google.com>
Commit-Queue: Qijiang Yūki Ishii <fqj@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index c11cbed..864a0c6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -723,8 +723,6 @@
     mojom_type_mappings_generator =
         "libchrome_tools/mojom_generate_type_mappings.py"
     sources = [
-      "mojo/public/cpp/base/big_buffer.typemap",
-      "mojo/public/cpp/base/big_string.typemap",
       "mojo/public/cpp/base/file.typemap",
       "mojo/public/cpp/base/file_error.typemap",
       "mojo/public/cpp/base/file_info.typemap",
@@ -732,9 +730,7 @@
       "mojo/public/cpp/base/memory_allocator_dump_cross_process_uid.typemap",
       "mojo/public/cpp/base/process_id.typemap",
       "mojo/public/cpp/base/read_only_buffer.typemap",
-      "mojo/public/cpp/base/ref_counted_memory.typemap",
       "mojo/public/cpp/base/shared_memory.typemap",
-      "mojo/public/cpp/base/string16.typemap",
       "mojo/public/cpp/base/text_direction.typemap",
       "mojo/public/cpp/base/thread_priority.typemap",
       "mojo/public/cpp/base/time.typemap",
@@ -756,8 +752,6 @@
       "mojo/public/interfaces/bindings/interface_control_messages.mojom",
       "mojo/public/interfaces/bindings/native_struct.mojom",
       "mojo/public/interfaces/bindings/pipe_control_messages.mojom",
-      "mojo/public/mojom/base/big_buffer.mojom",
-      "mojo/public/mojom/base/big_string.mojom",
       "mojo/public/mojom/base/file.mojom",
       "mojo/public/mojom/base/file_error.mojom",
       "mojo/public/mojom/base/file_info.mojom",
@@ -767,9 +761,7 @@
       "mojo/public/mojom/base/memory_allocator_dump_cross_process_uid.mojom",
       "mojo/public/mojom/base/process_id.mojom",
       "mojo/public/mojom/base/read_only_buffer.mojom",
-      "mojo/public/mojom/base/ref_counted_memory.mojom",
       "mojo/public/mojom/base/shared_memory.mojom",
-      "mojo/public/mojom/base/string16.mojom",
       "mojo/public/mojom/base/text_direction.mojom",
       "mojo/public/mojom/base/thread_priority.mojom",
       "mojo/public/mojom/base/time.mojom",
@@ -857,17 +849,12 @@
           "mojo/core/watcher_dispatcher.cc",
           "mojo/core/watcher_set.cc",
           "mojo/public/c/system/thunks.cc",
-          "mojo/public/cpp/base/big_buffer.cc",
-          "mojo/public/cpp/base/big_buffer_mojom_traits.cc",
-          "mojo/public/cpp/base/big_string_mojom_traits.cc",
           "mojo/public/cpp/base/file_info_mojom_traits.cc",
           "mojo/public/cpp/base/file_mojom_traits.cc",
           "mojo/public/cpp/base/file_path_mojom_traits.cc",
           "mojo/public/cpp/base/process_id_mojom_traits.cc",
           "mojo/public/cpp/base/read_only_buffer_mojom_traits.cc",
-          "mojo/public/cpp/base/ref_counted_memory_mojom_traits.cc",
           "mojo/public/cpp/base/shared_memory_mojom_traits.cc",
-          "mojo/public/cpp/base/string16_mojom_traits.cc",
           "mojo/public/cpp/base/text_direction_mojom_traits.cc",
           "mojo/public/cpp/base/thread_priority_mojom_traits.cc",
           "mojo/public/cpp/base/time_mojom_traits.cc",
@@ -924,7 +911,6 @@
           "mojo/public/cpp/platform/platform_channel_server_endpoint.cc",
           "mojo/public/cpp/platform/platform_handle.cc",
           "mojo/public/cpp/platform/socket_utils_posix.cc",
-          "mojo/public/cpp/system/buffer.cc",
           "mojo/public/cpp/system/data_pipe_drainer.cc",
           "mojo/public/cpp/system/data_pipe_producer.cc",
           "mojo/public/cpp/system/data_pipe_utils.cc",