clang: Remove -Wno-format now that our clang understands %as in c++.

BUG=none
TEST=builds
TBR=hans


Review URL: http://codereview.chromium.org/8983031

git-svn-id: http://src.chromium.org/svn/trunk/src/build@116397 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -1979,9 +1979,6 @@
               # This (rightyfully) complains about 'override', which we use
               # heavily.
               '-Wno-c++11-extensions',
-              # TODO(thakis): Reenable this once clang stops complaining about
-              # %as in format strings.
-              '-Wno-format',
             ],
             'cflags!': [
               # Clang doesn't seem to know know this flag.