Remove `Datamodel.open_asdf`

[braingram]

This method:

stdatamodels/src/stdatamodels/model_base.py

Lines 570 to 573 in 4041d5f

	def open_asdf(init=None,
	ignore_version_mismatch=True,
	ignore_unrecognized_tag=False,
	**kwargs):

duplicates (identical) options to asdf.open (which also uses ignore_version_mismatch=True and ignore_unrecognized_tag=False and is unused outside of the Datamodel class.

[braingram]

This is a bit more complicated than expected due to the selective keyword argument handling in open_asdf. The method does not pass kwargs to AsdfFile (but does to asdf.open) so several uses in stdatamodels (none in jwst) silently ignore things in kwargs.