Delete the remove_stale_autogen_files mechanism.

Previously when moving around directories for generated files, Dawn ran
into an issue where stale files where #included instead of the new ones,
causing compilation failures. To get around this a
remove_stale_autogen_files mechanism was added that scans the gen/
directory for files not in an allow-list of directories.

This mechanism is now causing problems for bringing up Dawn standalone
tests on Android as these test also generate files in Dawn's gen/
directories, and their files get deleted by remove_stale_autogen_files.

We are not foresseing any additional shuffling of directories and it's
safe to expect that all stale files have been removed from CI builder
caches at this time. So remove_stale_autogen_files can go. This is what
this CL does.

Fixed: dawn:1543
Change-Id: I7dbf1eae6c55b7659f3837b6d4a565052001ce57
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/103040
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>