gc plugin: stop recognizing old and unused options.

The options "enable-oilpan" and "warn-raw-ptr" are following r390631 no
longer being passed in as options, hence we can finally stop recognizing
them.

R=
BUG=585328

Review-Url: https://codereview.chromium.org/1941603002
Cr-Commit-Position: refs/heads/master@{#390875}
1 file changed