Remove Travis CI config

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.