Now that wasm code lives in its own native heap, we no longer need to
switch protection for the js code space. Hence, remove a left-over
CodeSpaceMemoryModificationScope.

Change-Id: I80830bc4b0eee672c9e5c7ba0088ffcbc5b2da57
Bug: v8:7549
Reviewed-on: https://chromium-review.googlesource.com/960002
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51899}