blink_gc_plugin: Remove handling for legacy-style naming.

The plugin option is left as a stub for now, to avoid the build from
failing. It will be removed once the gn files no longer need to pass
this argument.

Bug: 675879
Change-Id: Ib6c5da4e1d07d054704354367d12f8e34a50d41f
Reviewed-on: https://chromium-review.googlesource.com/474061
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#463852}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ae0304082bf05eeb62bd74b4ba9141f26cf1accb
184 files changed