MAINT: Remove deprecated requires_module

[larsoner]

We deprecated requires_module in MNE-Python, this cleans up some stuff with importorskip

[codecov]

Codecov Report

Merging #519 (1bc3fff) into main (5f005d7) will decrease coverage by 0.11%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main     #519      +/-   ##
==========================================
- Coverage   95.38%   95.28%   -0.11%     
==========================================
  Files          69       69              
  Lines        2885     2865      -20     
  Branches      412      412              
==========================================
- Hits         2752     2730      -22     
- Misses         63       65       +2     
  Partials       70       70              

Files Changed	Coverage Δ
mne_nirs/visualisation/tests/test_visualisation.py	100.00% <ø> (ø)
mne_nirs/conftest.py	96.49% <75.00%> (-3.51%)	⬇️
...e_nirs/experimental_design/_experimental_design.py	100.00% <100.00%> (ø)
mne_nirs/io/fold/tests/test_fold.py	100.00% <100.00%> (ø)
mne_nirs/io/snirf/tests/test_snirf.py	96.53% <100.00%> (-0.18%)	⬇️
mne_nirs/preprocessing/tests/test_mayer.py	100.00% <100.00%> (ø)