Remove direct support for PEP381 Mirrors

* PEP381 Mirroring support was never fully implemented leaving
  users of it trivially exploitable to a MITM or malicious mirror
  operator.
* 2 out of 6 of the mirrors have been removed from the pool and
  will never resolve.
* The remaining mirrors often fall behind
* The mirrors will likely never be available under HTTPS
* People who wish to use a mirror of PyPI can still do so by
  manually specifying a mirror url for --index-url or
  --extra-index-url which is more flexible, allowing for
  mirrors to be hosted under any domain.
Restore some functionality to --mirrors and issue a warning
Merge pull request #1098 from dstufft/remove-mirror-support

Remove direct support for PEP381 Mirrors