Remove obsolete JSFunction::IsOptimizable predicate.

Remove obsolete JSFunction::IsOptimizable predicate.

This just delegates to SharedFunctionInfo::optimization_disabled and
was primarily used for assertions. Removing it due to misleading name
because already optimized functions reported being "non-optimizable".

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1150683002

Cr-Commit-Position: refs/heads/master@{#28551}