CI: remove upstream compatibility checks (!1085)

Do not run upstream compatibility checks whenever pushing to master.
This is confusing, because if the upstream compatibility check fails,
then it appears that the last patch was broken in some way although it
isn't related.

I've moved the upstream compatibility checks to a separate repository
already, and added a badge to the pmaports gitlab project that indicates
whether they are currently succeding or not. The checks run hourly now.

Related: https://gitlab.com/postmarketOS/monitoring
Fixes: #457