Maps have a hard limit of 256 (non-inclusive) for the instance size in
words. For the native context object, we are very close to this upper
bound. This CL removes a few unused fields to give us a bit of
breathing room (parts of which I will use in a follow-up CL).

Bug: v8:5577
Change-Id: I096a45e47661f78f6bf23d71cbc29100e6e0592b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1706055
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62765}