[build] Remove last vestiges of gyp from MB configs

This cleans up mb_config.pyl and strips it from:
- remaining gyp_defines entries and other mention of gyp
- all redundant 'gn_' prefixes and configs

These changes are possible after https://crrev.com/c/911073

Bug: chromium:772804
Change-Id: I702405cc97ef2cc14b2c2cc5870c8315dde8342a
Reviewed-on: https://chromium-review.googlesource.com/912830Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51243}