Remove Travis CI config

[marshallward]

MOM6 stopped using the Travis CI many months ago, shortly after
migrating to GitHub CI.

The Travis CI config was retained in case other groups still preferred
to use it. But at this time, no groups appear to be doing so.

Also, the presence of this file can trigger runs in older forks which
may still keep Travis activated, which generally results in errors due
to the greater restrictions imposed by Travis.

To keep things running smoothly and avert such problems, this patch
simply removes the config file.

[codecov]

Codecov Report

Merging #143 (1df6b13) into dev/gfdl (cba297d) will not change coverage.
The diff coverage is n/a.

❗ Current head 1df6b13 differs from pull request most recent head 7bc1844. Consider uploading reports for the commit 7bc1844 to get more accurate results

@@            Coverage Diff            @@
##           dev/gfdl     #143   +/-   ##
=========================================
  Coverage     33.45%   33.45%           
=========================================
  Files           262      262           
  Lines         71436    71436           
  Branches      13340    13340           
=========================================
  Hits          23896    23896           
  Misses        43066    43066           
  Partials       4474     4474           

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