DEP: remove deprecated umfpack wrapper.  Closes gh-3002.

Executive summary:
- this is due to license issues, UMFPACK changed from a BSD-compatible license
  to GPL a long time ago.
- this solves install issues; SWIG was always an undocumented build-time
  dependency if UMFPACK was installed on a system.
- scikits.umfpack was lost, but @rc is about to revive it.
Merge pull request #3178 from rgommers/remove-umfpack

DEP: remove deprecated umfpack wrapper