MNT: Remove AFNI from smriprep docker container

[effigies]

It's possible that we're using AFNI through some niworkflows pipeline, but I don't think so. This will reduce the container size a little and build time by a bit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b5e8663) 53.47% compared to head (de41f41) 58.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##             next     #387      +/-   ##
==========================================
+ Coverage   53.47%   58.35%   +4.87%     
==========================================
  Files          24       27       +3     
  Lines        1840     1904      +64     
  Branches      239      239              
==========================================
+ Hits          984     1111     +127     
+ Misses        787      736      -51     
+ Partials       69       57      -12     

Flag	Coverage Δ
ds005	∅ <ø> (?)
ds054	45.92% <ø> (ø)
pytest	36.92% <ø> (?)

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