GH Actions: Remove cancel action. (#4843)

GH Actions: Remove cancel action. (#4843)

The previously introduced action to cancel running builds is not able to
cancel builds on other branches, only on the same branch. As a consequence,
when pushing to a branch for which a PR has been submitted, builds on the
main repository are not cancelled.

This removes the cancel build. If we want behavior similar to how it was
on Travis, we need a workaround / more sophisticated solution since GH
Actions doesn't really allow / support this (due to permission issues).