Now that the embedder side supports v8::kGCCallbackFlagCollectAllAvailableGarbage,
this CL removes the old v8::kGCCallbackFlagForced from Heap::CollectAllAvailableGarbage.

BUG=591463
LOG=Y

Review URL: https://codereview.chromium.org/1762313002

Cr-Commit-Position: refs/heads/master@{#34517}