Remove travis configuration files.

After porting the travis jobs to github
actions, it doesn't make a lot of sense
to keep travis around, especially since
we have come up with a host of problems
with it, including, but not limited to:

* builds timing out on a regular basis
* builds failing because of travis'
  networking problems
* contributors not having rights to restart
  failing jobs corresponding to their PRs
* misconfiguration of the build pipeline
  causing CI runs taking a whole load more
  than they need to.

Author: NlightNFotis