We had two implementations of a thread-local counter for the number of
writers: One in {CodeSpaceWriteScope} and one in
{WasmCodeManager::SetThreadWritable}. This CL removes the latter, and
uses the counter in {CodeSpaceWriteScope} for all implementations.

R=jkummerow@chromium.org

Bug: v8:11974
Cq-Include-Trybots: luci.v8.try:v8_mac_arm64_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_mac_arm64_dbg_ng
Change-Id: I683131296c6106a2b12986942bb18e6c0e716612
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3024148Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75733}