Remove support for [LegacyImplementedInBaseClass] IDL extended attribute

Remove support for [LegacyImplementedInBaseClass] IDL extended attribute now
that the refactoring is complete.

R=haraken, nbarth@chromium.org
BUG=344224
NOTRY=true

Review URL: https://codereview.chromium.org/169273002

git-svn-id: svn://svn.chromium.org/blink/trunk@167280 bbb929c8-8fbe-4397-9dbb-9b2b20218538