Remove healpix_to_image function (moved to reproject repo)

@udaraabeysekara tried to use the `healpix_to_image` function from Gammapy, but it's broken.
This pull request removes it from Gammapy.

Hopefully a fully functional and working version will be available in the `reproject` repo ... there's an open pull request here (that needs some more work):  https://github.com/astrofrog/reproject/pull/27