[icu] Use char16_t on non-Windows platforms

Following https://crrev.com/c/2599088 Chromium no longer requires UChar
and base::char16 to be exactly the same type. This now allows dropping
the UCHAR_TYPE override for non-Windows platforms, making char16_t the
default UTF16 character type.

Bug: chromium:911896
Change-Id: I5d1e6dc644c16cc31318ea9d9cf89b114900e061
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/2617781
Reviewed-by: Frank Tang <ftang@chromium.org>
diff --git a/icu.gyp b/icu.gyp
index bb54b83..7ec9009 100644
--- a/icu.gyp
+++ b/icu.gyp
@@ -44,10 +44,6 @@
           'UCHAR_TYPE=wchar_t',
         ],
 	'cflags': [ '/utf-8' ],
-      },{
-        'defines': [
-          'UCHAR_TYPE=uint16_t',
-        ],
       }],
       ['(OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris" \
          or OS=="netbsd" or OS=="mac" or OS=="android" or OS=="qnx") and \