Remove instruction to run optional tests

[guarani]

Fixes https://github.com/wordpress-mobile/release-toolkit-gutenberg-mobile/pull/72

Optional tests are now run by default on Gutenberg Mobile release PRs both platforms:

• https://github.com/wordpress-mobile/WordPress-Android/pull/15449
• https://github.com/wordpress-mobile/WordPress-iOS/pull/16233 So we can now remove the instruction here that prompted release wranglers to manually run these tests.

[guarani]

cc @geriux and @antonis since this change, if it lands before the 1.64.0 release checklist is posted, will slightly affect the checklist that's used.