Android: Remove obsolete "verify_manifest" GN variable

Bug: 1095654
Change-Id: Iaf820c43e56f3f8680c34981b9d1bc3cb70b6284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2257732


Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780845}