has_prototype_slot on Context::FunctionMapIndex() is unused in its
implementation, and all callers passes just true.
Whether the function has prototype or not is determined by FunctionKind
instead.

Change-Id: I598515b25267dd3470cccbf3236b5e64c6959d00
Reviewed-on: https://chromium-review.googlesource.com/c/1469542Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59584}