HOTFIX: Remove generated-mima-excludes file after runing MIMA.

This has been causing some false failures on PR's that don't merge
correctly.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #971 from pwendell/mima and squashes the following commits:

1dc80aa [Patrick Wendell] HOTFIX: Remove generated-mima-excludes file after runing MIMA.