The %GetPrototype runtime function is not used anymore. Also remove the
cctests that were introduced to guard the Crankshaft optimizations for
the %_GetPrototype intrinsic.

Bug: v8:8015
Change-Id: I4b848f2c8d67209dae002d260a26867299d6b4a5
Reviewed-on: https://chromium-review.googlesource.com/1199106Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55541}