SH_DONT_PRUNE_UNUSED_FUNCTIONS was only used by tests, and never used by
ANGLE or chromium.

Bug: angleproject:4889
Change-Id: I4926f86125e69b07e9d4d95134b7b70522e0d64f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2607491Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>