Remove unnecessary clang flags for ICU

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 0ad2539bd86bc1ae14b1b8a67669b7375a2b701a..351fbb10a0bb413e241e1344b4fef46538927d83 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -290,19 +290,6 @@ component("icui18n") {
       "-Wno-deprecated-declarations",
     ]
   }
-  if (is_clang) {
-    # uspoof.h has a U_NAMESPACE_USE macro. That's a bug,
-    # the header should use U_NAMESPACE_BEGIN instead.
-    # http://bugs.icu-project.org/trac/ticket/9054
-    configs -= [ "//build/config/clang:extra_warnings" ]

[Nico, 2015/05/28 02:33:51]

I'm not sure if this is right. The comment refers to -Wno-header-hygiene below;
the comment is in the wrong place.

But if ICU builds without disabling extra_warnings, I suppose that's a Good
Thing :-)

[jungshik at Google, 2015/05/28 18:07:13]

builds/config/clang/BUILD.gn has the following (I guess that's the only place
where extra_warnings is defined). |extra_warnings| has '-Wheader-hygiene' and
'-Wstring-conversion'. So, the comment placement makes sense (although one has
to go look at what |extra_warnings| has to understand the comment ;-) ). 

config("extra_warnings") {
  cflags = [
    "-Wheader-hygiene",

    # Warns when a const char[] is converted to bool.
    "-Wstring-conversion",
  ]

-
-    cflags += [
-      "-Wno-header-hygiene",
-
-      # Looks like a real issue, see http://crbug.com/114660
-      "-Wno-return-type-c-linkage",
-    ]
-  }
 }
 
 component("icuuc") {