Removal of x-shared polar axes causes crash

[anntzer]

Bug report

Bug summary

All's in the title.

(Note that x-sharing (which really means theta-sharing) polar axes makes sense because one can use set_thetalims to draw just a wedge.)

Code for reproduction

from pylab import *
ax1, ax2 = gcf().subplots(2, sharex=True, subplot_kw={"projection": "polar"}); ax2.remove()

Actual outcome

Traceback (most recent call last):
  File "<string>", line 4, in <module>
  File ".../path/to/matplotlib/artist.py", line 162, in remove
    self._remove_method(self)
  File ".../path/to/matplotlib/figure.py", line 967, in delaxes
    _reset_locators_and_formatters(last_ax.xaxis)
  File ".../path/to/matplotlib/figure.py", line 930, in _reset_locators_and_formatters
    isDefault = majloc.axis.isDefault_majloc
AttributeError: '_AxisWrapper' object has no attribute 'isDefault_majloc'

Expected outcome

Normal axes removal.

Matplotlib version

• Operating system: linux
• Matplotlib version (import matplotlib; print(matplotlib.__version__)): head
• Matplotlib backend (print(matplotlib.get_backend())): mplcairo
• Python version: 39
• Jupyter version (if applicable):
• Other libraries:

#13482 happens to fix this, because it moves handling of isDefault_* to the tickers themselves (where they logically belong), rather than having them on the axis.

(Note that this is a separate issue from #19989 as the root cause seems very different.)

[anntzer]

feel free to remilestone