Delete the unnecessary ICU header files

core.gypi file is updated accordingly, too. core depends on wtf, which
pulls in icu dependencies.

BUG=232922
TEST=blink builds just fine.

Review URL: https://chromiumcodereview.appspot.com/18162010

git-svn-id: svn://svn.chromium.org/blink/trunk@154145 bbb929c8-8fbe-4397-9dbb-9b2b20218538