Remove CXX_VARIADIC_TEMPLATES compiler feature flag

This flag is never used and variadic templates are now allowed on all the supported platforms.

Review URL: https://codereview.chromium.org/613203003

git-svn-id: svn://svn.chromium.org/blink/trunk@183018 bbb929c8-8fbe-4397-9dbb-9b2b20218538