dt.weekday_name - removal of function

[nameloCmaS]

Expanding on #8610, I considered the below in deciding to remove the parameter dt.weekday_name:

1. The parameter has not been able to be used since v0.17.0 (bump the dependencies #4942) which is when pandas v1.0.0 was set as the minimum required version. This is 3 years it has not worked.
2. No one has raised an issue re this in 3 years, so assume it is not used or required by the user base; thus it is unlikely to break any code.
3. It was not in the test suite.
4. There are more flexible ways to get date strings such as strftime() which account for localisation.

I considered a number of solutions with the above:

1. Add a warning and return an empty T_DataArray.
2. Return the weekday_name using strftime() or similar.
3. Remove the parameter and implement the pandas day_name method (but this isn't in cftime).

It should also be noted that the parameter does not work for both pandas.dt and cftime:

import xarray as xr
import dumpy as np
dates = xr.cftime_range(start="0001", periods=2, freq="MS", calendar="noleap")
da = xr.DataArray(dates)
da.dt.weekday_name

returns:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/sam/miniconda3/envs/xarray-docs/lib/python3.10/site-packages/xarray/core/accessor_dt.py", line 462, in weekday_name
    return self._date_field("weekday_name", object)
  File "/Users/sam/miniconda3/envs/xarray-docs/lib/python3.10/site-packages/xarray/core/accessor_dt.py", line 245, in _date_field
    result = _get_date_field(_index_or_data(self._obj), name, dtype)
  File "/Users/sam/miniconda3/envs/xarray-docs/lib/python3.10/site-packages/xarray/core/accessor_dt.py", line 139, in _get_date_field
    out = access_method(values, name)
  File "/Users/sam/miniconda3/envs/xarray-docs/lib/python3.10/site-packages/xarray/core/accessor_dt.py", line 65, in _access_through_cftimeindex
    field_values = getattr(values_as_cftimeindex, name)
AttributeError: 'CFTimeIndex' object has no attribute 'weekday_name'

• Closes dt.weekday_name - attribute removed from Pandas v1.0.0 #8610
• Tests unaffected
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• Removed functions/methods from api.rst and api-hidden.rst

[max-sixty]

This is 3 years it has not worked.

OK, that's fairly persuasive!

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Thanks @nameloCmaS !