Drop the dead num_threads input from MCFLIRT2ITK

[asaraog]

Closes #1042. Nipype sets num_threads on any interface that declares it, so the deprecation warning always fired; MCFLIRT2ITK never used the value.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.15%. Comparing base (09d0d2b) to head (0a0609c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1060   +/-   ##
=======================================
  Coverage   67.14%   67.15%           
=======================================
  Files          54       54           
  Lines        6855     6850    -5     
  Branches      853      852    -1     
=======================================
- Hits         4603     4600    -3     
+ Misses       2038     2037    -1     
+ Partials      214      213    -1     

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[effigies]

Seems right.