Since the original SPIR-V is never store after being modified, we don't
need to keep the transformed SPIR-V either.

Bug: angleproject:4513
Test: CQ
Change-Id: I4b70cfa355a08107e6a27377f6258cbf9e4072be
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2270718
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>