ci: Drop Android API 21 build jobs

API version 21 was used for Android 5.0
(https://developer.android.com/studio/releases/platforms#5.0), which is
no longer supported
(https://en.wikipedia.org/wiki/Android_version_history).

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
12 jobs for wip/pwithnall/ci-cleanups in 15 minutes and 10 seconds (queued for 2 seconds)