Remove the ProgramExecutable::mProgram[Pipeline]State pointers.

Bug: angleproject:4520
Test: Build/CQ
Change-Id: I1717e291ff9beec226bd2888e990b27d8078797c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2243764
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>