RF: Upgrade nitransforms and remove workarounds

[effigies]

I've manually verified that load_ants_h5() can be replaced with nitransforms.manip.load() and that this does the right thing, and that using nitransforms.apply(xfm, moving, target) does indeed get me basically the same thing as ANTs.

[codecov]

Codecov Report

Attention: Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 72.06%. Comparing base (6e6eadc) to head (7a7a623).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
fmriprep/workflows/bold/registration.py	0.00%	4 Missing ⚠️
fmriprep/utils/transforms.py	0.00%	2 Missing ⚠️
fmriprep/interfaces/resampling.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3378      +/-   ##
==========================================
+ Coverage   71.70%   72.06%   +0.35%     
==========================================
  Files          57       57              
  Lines        4259     4231      -28     
  Branches      640      456     -184     
==========================================
- Hits         3054     3049       -5     
+ Misses       1090     1067      -23     
  Partials      115      115              

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[mgxd]

LGTM