remove last mentions of, and vestigial files relating to, Travis CI

[Pomax]

Closes #6640

• removes vestigial files
• updates the docs to no longer mention Travis
• updates mergify to tap into the Github Actions CI tasks instead of the old Travis task

[danielfmiranda]

Hi Pomax,

Thank you for taking the time to do this, I can see it being very helpful in the future whenever someone looks through the readme or the workflow document. I gave all the changed files a look, and everything looks good to me!

Approving!