Pull NIfTI headers from existing host

[Lestropie]

Following on from prior comment in #2859.

Rather than making a repository in the MRtrix3 organisation to store the header files for the NIfTI image format, there may instead be existing repositories from which those files could be pulled during configuration as third-party dependencies as per #2859, and therefore removed from this repository:

https://nifti.nimh.nih.gov/pub/dist/src/nifti2/nifti1.h
https://nifti.nimh.nih.gov/pub/dist/src/nifti2/nifti2.h

[daljit46]

Closed by #2979.