The "Program" part of ProgramState::getProgramExecutable() is already
implied by being part of a ProgramState, so it can be removed.

Bug: angleproject:3570
Change-Id: I35ffb2af81196fa7f189f7d3a37158f5f1951141
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2209317Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>