The {remaining_uncommitted_code_space} method is only used for testing.
This CL removes it, and replaces all uses by {committed_code_space}.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: Icb50471da3564a5cd114b15836c8b346b932a108
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1559735
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60704}