MAINT: Remove unnecessary check from before swapaxes (API change)

This check was a speed optimization in the numpy code. Due to
a small change in the MapIter API, the check is now wrong in
corner cases and becomes fully unnecessary.
The previous API had not been public in any release numpy
version.
Merge pull request #1378 from seberg/patch-1

MAINT: Remove unnecessary check from before swapaxes (API change)