Remove HTTP Basic-Auth repository support

[sheepman4267]

This completely removes support for authenticating to eopkg repositories using HTTP Basic-Auth.
Closes #108.

Rationale

1. This isn't something we use or have any reason to use officially.
2. No one on the team has a worthwhile test case for this functionality.
3. eopkg has an abundance of features which are rarely or never used, and the codebase is very difficult to maintain. Removing unused or impractical non-features such as this reduces code complexity and test surface area.

Test Plan

1. Check out this PR.
2. Try eopkg --username=someuser <command> and note that you now receive an error.
3. Do the same test with --password and the shorthand arguments -p and -u.
4. Try some basic operations requiring fetches from the server (install, remove, update, fetch, etc.) normally and see that they still work.