Remove deprecated code from wcs

[eerovaher]

Description

The first commit removes code deprecated in #10229. The users are meant to import things from astropy.wcs.wcsapi and apparently the internals of that module are considered private. The deprecation therefore had no change log entry, and the removal of the code shouldn't have an entry either.

The second commit removes code deprecated in #6682. There were no change log entries announcing the deprecation, so likewise there's no entry announcing the removal.

There is one more deprecated function:

astropy/astropy/wcs/wcs.py

Lines 966 to 972 in 98d82dd

	def _read_d2im_old_format(self, header, fobj, axiscorr):
	warnings.warn(
	"The use of ``AXISCORR`` for D2IM correction has been"
	" deprecated.`~astropy.wcs` will read in files with ``AXISCORR`` but"
	" ``to_fits()`` will write out files without it.",
	AstropyDeprecationWarning,
	)

But in this case I'm not sure if removing the deprecated function is the right thing to do. Maybe the warning type should be changed instead. I'd prefer if someone more familiar with wcs would have a look at this.

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[pllim]

Needs a change log. Thanks!

[pllim]

Still need a change log.

[eerovaher]

I'd first like to know what should be done with _read_d2im_old_format().

[pllim]

This? Yeah, not sure. Looks like the warning is always emitted if this gets called. Maybe @mcara would know. But this can also be a follow-up PR because I feel like bigger changes are needed to get rid of this warning.

    def _read_d2im_old_format(self, header, fobj, axiscorr):
        warnings.warn(
            "The use of ``AXISCORR`` for D2IM correction has been"
            " deprecated.`~astropy.wcs` will read in files with ``AXISCORR`` but"
            " ``to_fits()`` will write out files without it.",
            AstropyDeprecationWarning,
        )

[eerovaher]

To elaborate: from the deprecation warning in _read_d2im_old_format() it is not clear to me that reading files with AXISCORR is or even should be deprecated, so it's not obvious to me that this function should be removed. Should it be the case that the warning type needs to be changed, but doing that is not trivial, then it would indeed be a good idea to leave it for a dedicated pull request, but I'd still like that to be decided here.

[eerovaher]

I haven't heard anything about _read_d2im_old_format(), but that shouldn't prevent the removal of other deprecated code. As far as I can tell the file that actually defines SlicedLowLevelWCS is not considered to be part of the public API and there was no change log entry for deprecating the old file, so I haven't added a change log entry for its removal either. For NoConvergence **kwargs I added a change log entry like @pllim requested. I also rebased the pull request to pick up the RTD configuration update from yesterday.

[pllim]

LGTM now. Thanks!