Build/Test Tools: Remove the TravisCI configuration file.

In [49162], GitHub Action workflow configuration files were introduced to run all of Core’s automated testing with the intent to fully transition after some time was allowed for testing.

After two full months of testing, the time to finish this transition has come.

We thank TravisCI for testing the codebase through nearly 20 major and many more minor releases.

Merges [49876] to the 4.7 branch.
See #52161. See #50401.

git-svn-id: https://develop.svn.wordpress.org/branches/4.7@49886 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj