Remove unrar from oss-fuzz (#14259)

This currently depends on Chrome engineers mirroring unrar to
https://github.com/aawc/unrar.git

Chrome has begun fuzzing the copy of unrar in the chromium repo, so we
do not plan to do this mirroring going forward.

Co-authored-by: Daniel Rubery <drubery@chromium.org>

Author: drubery