Remove unnecessary call to the deprecated g_thread_init().

g_thread_init() has been deprecated since glib 2.32. This CL removes the
unnecessary call to g_thread_init(), so that we can later migrate to
glib 2.34.

BUG=chromium:253025
TEST=Tested the following:
1. Build and run unit tests.
2. Run the following tests:
   - platform_CrosDisksDBus
   - platform_CrosDisksFilesystem
   - platform_CrosDisksArchive
   - platform_CrosDisksFormat

Change-Id: I1df405d9b30737f89574012369d06ccd52fb5a9f
Reviewed-on: https://gerrit.chromium.org/gerrit/59681
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/main.cc b/main.cc
index a821cd2..d3258b4 100644
--- a/main.cc
+++ b/main.cc
@@ -99,7 +99,6 @@
 
 int main(int argc, char** argv) {
   g_type_init();
-  g_thread_init(NULL);
 
   CommandLine::Init(argc, argv);
   CommandLine* cl = CommandLine::ForCurrentProcess();