fix: some lgtm errors

[mgxd]

Fixes #2475

Addresses some of the issues in
https://lgtm.com/projects/g/nipy/nipype/alerts/?mode=tree&severity=error

This includes the removal of:

• nipype.pipeline.plugins.ipythonx
• nipype.interfaces.nipy.FmriRealign4d
• nipype.interfaces.ants.antsCorticalThickness
• nipype.interfaces.ants.antsBrainExtraction

[satra]

if out_files is None

[satra]

@mgxd - can you merge with master

[codecov-io]

Codecov Report

Merging #2478 into master will increase coverage by 0.01%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master    #2478      +/-   ##
==========================================
+ Coverage   66.66%   66.67%   +0.01%     
==========================================
  Files         328      327       -1     
  Lines       42529    42417     -112     
  Branches     5278     5262      -16     
==========================================
- Hits        28350    28280      -70     
+ Misses      13500    13457      -43     
- Partials      679      680       +1

Flag	Coverage Δ
#smoketests	50.83% <23.52%> (+0.04%)	⬆️
#unittests	64.01% <40%> (+0.04%)	⬆️

Impacted Files	Coverage Δ
nipype/pipeline/plugins/__init__.py	100% <ø> (ø)	⬆️
nipype/info.py	85.07% <ø> (-0.22%)	⬇️
nipype/interfaces/nipy/preprocess.py	48.27% <ø> (+0.71%)	⬆️
nipype/interfaces/ants/registration.py	73.18% <ø> (+0.09%)	⬆️
nipype/pipeline/engine/base.py	88.46% <ø> (+4.82%)	⬆️
nipype/pipeline/engine/utils.py	82.24% <0%> (-0.1%)	⬇️
nipype/interfaces/cmtk/cmtk.py	18.65% <0%> (-0.04%)	⬇️
nipype/utils/filemanip.py	77.59% <0%> (+1.46%)	⬆️
nipype/pipeline/plugins/ipython.py	22.82% <0%> (-0.51%)	⬇️
nipype/interfaces/cmtk/nx.py	19.24% <0%> (ø)	⬆️
... and 8 more

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