Delete `clinica.utils.mri_registration` module

Description

The module clinica.utils.mri_registration only contains a single function convert_flirt_transformation_to_mrtrix_transformation which is only used in a single place in the code base:

clinica/clinica/pipelines/dwi/connectome/pipeline.py

Line 353 in 7d2819a

function=convert_flirt_transformation_to_mrtrix_transformation,

Steps

I propose that we delete this useless module and move the function to clinica.pipelines.dwi.connectome.utils.
Since it is used as a Nipype interface, it would be great to implement the corresponding adapter in clinica.pipelines.dwi.connectome.tasks.
Finally, it would be awesome to implement some unit tests for it since they are currently missing.

Hints for the unit testing:
The function is basically doing a subprocess call, so we could refactor a bit to have a helper function building and returning the command that should be run (that would be easy to test).
Then, by monkey patching check_software and os.system we could test the behavior of the function without running the underlying MRTrix command.