docs: remove homebrew tap instructions
docs: remove homebrew tap instructions

These instructions provide a way to compile MRtrix3 from source, which
we expect will be obsolete by the time binary installers are available.
But they also don't provide the user with a functional install for
development.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3