Remove RuntimeEnabledFeatures flag MediaQueryParser (status=stable)

This flag has been stable for over three months, no point keeping it around.

BUG=375171

Review URL: https://codereview.chromium.org/559903002

git-svn-id: svn://svn.chromium.org/blink/trunk@181744 bbb929c8-8fbe-4397-9dbb-9b2b20218538