We don't need to have an (untested) fallback for the case that a
prototype map is not stable when specializing a named property,
because prototype maps are always stable (which is now guarded
by a DCHECK in CompilationDependencies).  Less (dead) code is
better code.

R=verwaest@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1417973006

Cr-Commit-Position: refs/heads/master@{#31524}