[es6] Remove left-overs from Function.prototype.toMethod.

[es6] Remove left-overs from Function.prototype.toMethod.

The actual Function.prototype.toMethod was removed some time already,
but there were some stuff (esp. %ToMethod) left in the tree, including
tests for %ToMethod.  This code (and esp. the tests) cause trouble in
the process of moving bound functions away from JSFunction; so since
the code is unused anyway, we can as well remove it.

The original removal of Function.prototype.toMethod was in February
2015 in 68e489758607bb7373409b49a0a7883407a322b3.

R=jarin@chromium.org
BUG=v8:3330
LOG=n

Review URL: https://codereview.chromium.org/1366063002

Cr-Commit-Position: refs/heads/master@{#30925}