Remove travis configuration files.

[NlightNFotis]

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.

After this is merged, the aim is to slowly phase out the codebuild jobs as well.

NOTE: This should be merged after the last travis check (building against debug libstdc++ has been merged).

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[martin-cs]

Sounds good as long as the external access to the logs of the other build systems continues to work.

[hannes-steffenhagen-diffblue]

Good job on removing trace files/mentions and also documenting why we’re doing this!

[codecov]

Codecov Report

Merging #5533 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5533   +/-   ##
========================================
  Coverage    68.52%   68.52%           
========================================
  Files         1187     1187           
  Lines        98265    98265           
========================================
  Hits         67339    67339           
  Misses       30926    30926           

Flag	Coverage Δ
#cproversmt2	42.96% <ø> (ø)
#regression	65.68% <ø> (ø)
#unit	32.26% <ø> (ø)

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