Remove redundant test: COMPILER(GCC) || COMPILER(CLANG) => COMPILER(GCC)

COMPILER(CLANG) =implies=> COMPILER(GCC)
so
COMPILER(GCC) || COMPILER(CLANG) == COMPILER(GCC)

Remove redundant and misleading test.

Came up in:
[POSSIBLE PERFORMANCE IMPACT] Use __builtin_expect on Clang as well (for UNLIKELY and LIKELY)
https://codereview.chromium.org/114933002/
NOTRY=true

Review URL: https://codereview.chromium.org/105673006

git-svn-id: svn://svn.chromium.org/blink/trunk@163876 bbb929c8-8fbe-4397-9dbb-9b2b20218538