Remove unnecessary virtual specifiers Clang has added a new warning for virtual specifiers on methods of `final` classes. This cleans them up in preparation for enabling that warning. Bug: chromium:403236787 Change-Id: I7acee9b2b99f13279d50ebe2cef832e734792546 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6388035 Reviewed-by: Yuxin Hu <yuxinhu@google.com> Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>