Moves CallStubR to be private and drop the return_count argument from
CallStub and its callchain, and instead use the GetReturnCount on the
call descriptor.

Also removes unused Retain function from code-assembler.

BUG=v8:6949,v8:11074

Change-Id: Ic0ebc72f84c2eab156c545af56237d4c46548c05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2523324
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71038}