Drop deprecated sudo: option in TravisCI config

[PeterDaveHello]

That config was deprecated for a while, reference:

• https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures

[ljharb]

What that specifically says is:

if you use a specific build environment group and do not specify sudo: required, it’s possible you’ll see slight variations in your build environment as projects are migrating.

In other words, sudo: false was deprecated, not sudo: required - without the explicit config value, travis applies a heuristic to determine if sudo is available. I'm not sure the tests can pass without it.

[PeterDaveHello]

Looks like the quote was just part of the "Migration Timeline", but the migration was finished already, there shouldn't be any difference about specified sudo: or not?,

[ljharb]

I've gotten confirmation from travis that sudo: required is indeed a noop, so this can land. Thanks!