Remove basic authentication support for GitHub.

[jonchang]

Since we (and GitHub) no longer support password authentication using non-token passwords, always set the Authorization header, rather than needlessly checking for a (possibly incorrect) username. This also simplifies the code.

Fixes issue identified in Homebrew/discussions#273 (comment).

Tested on brew pr-pull --verbose and confirmed that it behaves as expected (not passing --user foo:*** and instead passing --header Authorization: token *** to curl)

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[BrewTestBot]

Review period will end on 2020-12-18 at 12:18:35 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Thanks @jonchang!