Remove unused flag from the feature.gypi

Two flags - ENABLE_CSS_EXCLUSIONS and ENABLE_CSS_REGIONS - are not used.
Those features use runtime flag rather than compile flag currently.

Review URL: https://codereview.chromium.org/52103003

git-svn-id: svn://svn.chromium.org/blink/trunk@160998 bbb929c8-8fbe-4397-9dbb-9b2b20218538