Remove TRACE_BACKGROUND_GC which was used to emit events on the
background thread. It is replaced by TRACE_GC1, which uses the ThreadKind
argument to work both on main and background threads.

This CL also removes the dedicated BackgroundScope enum, all scopes
are now in GCTracer::Scope.

Change-Id: Ie377082d6a278dd46f2fa359611fdd99a08afcea
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2560203Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71458}