Now TFJ builtins can use their own descriptors so there's no need to
keep the hacky BuiltinDescriptor around.

Bug: v8:7754
Change-Id: Ia7f23a21fb979370fd2149fef13186b83a3d5d30
Reviewed-on: https://chromium-review.googlesource.com/1104428
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53806}