Apply suggestions from code review

Author: effigies

nipype/interfaces/mrtrix3/preprocess.py

@@ -284,7 +284,6 @@ class DWIPreprocInputSpec(MRTrix3BaseInputSpec):
     )
     pe_dir = traits.Str(
         argstr="-pe_dir %s",
-        mandatory=False,
         desc="Specify the phase encoding direction of the input series, can be a signed axis number (e.g. -0, 1, +2), an axis designator (e.g. RL, PA, IS), or NIfTI axis codes (e.g. i-, j, k)",
     )
     ro_time = traits.Float(

nipype/interfaces/mrtrix3/tests/test_auto_DWIPreproc.py

@@ -91,7 +91,6 @@ def test_DWIPreproc_inputs():
         ),
         pe_dir=dict(
             argstr="-pe_dir %s",
-            mandatory=False,
         ),
         ro_time=dict(
             argstr="-readout_time %f",