Framework: Remove unused Travis install dependencies

[aduth]

Previously: #17004

This pull request seeks to remove the installation step of two seemingly unused dependencies from the Travis configuration install stage. There are no other references to either dotenv or wait-on anywhere in Gutenberg or wordpress-develop, so as best I can tell, they should not be necessary. My assumption is that they were considered in early iterations of #17004 as an alternative to the sleep 10 command which follows a few lines after the removed line.

Testing Instructions:

Travis build should pass.

[aduth]

There are no other references to either dotenv or wait-on anywhere in Gutenberg or wordpress-develop

Turns out I was searching the built copy of WordPress, not wordpress-develop. There are indeed a few references to each in tools/local-env of the wordpress-develop source we copy a few lines above this in the install step (example).

It's a bit awkward that we satisfy these dependencies this way (vs. installing based on the copied wordpress-develop-master/package.json), but it at least proves to be the case they are indeed required.

I've restored the removed line, but also think it would be wise to include additional comments for future maintainers who find themselves in a similar state of confusion as myself. I've expanded on the existing comment in f609dc6.

[gziolo]

If this is meant to include the code comment after all, it's good to go :)

[aduth]

Yep, only intending to include the clarifying comment after discovering the purpose by which it's needed. Thanks!