Remove unused cls argument to just_the_step_from_cmdline

[braingram]

just_the_step_from_cmdline is used in 2 places (outside of stpipe unit tests):

• in jwst https://github.com/spacetelescope/jwst/blob/faaf4929c4d08c5a670b3a924072ff01bf66967e/jwst/stpipe/tests/test_step.py#L65 (where cls is not provided)
• in stpipe

  stpipe/src/stpipe/cmdline.py

  Line 384 in fc937a2

  step, step_class, positional, debug_on_exception = just_the_step_from_cmdline(

  (where cls is passed in from step_from_cmdline)

stpipe/src/stpipe/cmdline.py

Line 159 in fc937a2

def just_the_step_from_cmdline(args, cls=None):

Removing step_script and step_from_cmdline likely makes sense as these could provide a cls (if they were used).