Only FullCodegen code ever gets flushed by code flushing. Since we are
deprecating the old pipeline, the added complexity introduced by code
flushing is no longer worth it. This CL removes it (but keeps code aging,
which is used to unlink SFIs from the compilation cache).

BUG=v8:6389,v8:6379,v8:6409

Change-Id: I90de113a101f86dbeaaf0511c61a090ef12aa365
Reviewed-on: https://chromium-review.googlesource.com/507388
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45446}