The member ProgramPipelineState::mHasBeenBound is unused and should be
removed.

Bug: angleproject:3570
Test: CQ
Change-Id: Idc41574f79e665ac36bef5b206704c667c49dadb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2301101Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>