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With the updated GC clang plugin that rolled out as part of issue 604993,
we no longer need to supply enable-oilpan to it. Stop doing so along
with removing the enable_oilpan configuration option entirely.

R=haraken,jochen
BUG=585328

Review-Url: https://codereview.chromium.org/1930913002
Cr-Commit-Position: refs/heads/master@{#390640}