factor: delete unused api code

[StuartRucker]

Deleted code and tests for /api/repo and for POST /api/buid, along with accompanying test cases

An additional small fix to throw an error when repoid is null when posting data. This error was because decodeURIComponent of a null value returns an 'undefined' string.

Also, no longer checks if repo is private.

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[bcoe]

I believe this got a little screwed up during the process of resolving conflicts, let's not merge yet.

[bcoe]

closing in favor of: https://github.com/GoogleCloudPlatform/flaky-service/pull/272