The portability tests cannot be run on every PR.

Instead we need to run build, doc-build on every PR (just as we already run lint)

We also remove the ticket workflow; it is not needed.

Fixes sagemath/trac-to-github#157

CC: @tobiasdiez

Component: porting

Author: Matthias Koeppe

Branch/Commit: u/mkoeppe/gh_actions__remove_pull_request_triggers_for_portability_workflows @ 59033e2

Reviewer: Tobias Diez

Issue created by migration from https://trac.sagemath.org/ticket/34942