The go command previously probed bitbucket servers in case of a 403 status to figure out which VCS command (git or hg) to use to fetch repositories hosted on Bitbucket.

Bitbucket no longer supports hg (https://bitbucket.org/blog/sunsetting-mercurial-support-in-bitbucket), and per discussion on CL 371720 they would like to change the status code returned by their servers — an implementation detail currently hard-coded by go get.

Since the choice of VCS tool is no longer ambiguous, we should remove the VCS probing logic. That will also make the go command's behavior for private Bitbucket repos more closely match its behavior for other private repos in general.