Remove gammapy.image.calc_footprint

Remove [gammapy.image.calc_footprint](https://gammapy.readthedocs.org/en/latest/api/gammapy.image.calc_footprint.html) once Astropy 0.4 has been released.

This functionality has been implemented in Astropy:
https://github.com/astropy/astropy/pull/2348
http://docs.astropy.org/en/latest/api/astropy.wcs.WCS.html?#astropy.wcs.WCS.calc_footprint