Now that we no longer use code specialization to patch WasmCode after it
was generated, we no longer need to flush the instruction cache after
instance building finishes.

R=clemensh@chromium.org

Change-Id: I4ce31876dde9c6dcc4ad945e024641b72330fe4f
Reviewed-on: https://chromium-review.googlesource.com/1128975Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54333}