This removes the usage of retaining_path_mode template parameter in
main thread marking visitor and makes the incremental visitor
identical to the stop-the-world visitor.

Subsequent CL will switch these visitors to MarkingVisitorBase.

Bug: chromium:1019218
Change-Id: I00e3ef190d8f2ac821cb63b02c402aad5ea49e9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1897538Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64757}