Remove more NaN in categories checking

[TomAugspurger]

These can probably all be removed / simplified now that we don't allow NaN in the categories.

• pandas/pandas/core/arrays/categorical.py

  Line 1353 in 38afa93

  obj = (self.remove_categories([np.nan]) if dropna and

• pandas/pandas/core/arrays/categorical.py

  Line 1326 in 38afa93

  if isna(result.categories).any():

• pandas/pandas/core/arrays/categorical.py

  Line 1279 in 38afa93

  if np.nan in self.categories:

• pandas/pandas/core/arrays/categorical.py

  Lines 1892 to 1898 in 38afa93

  	# FIXME: the following can be removed after GH7820 is fixed:
  	# https://github.com/pandas-dev/pandas/issues/7820
  	# float categories do currently return -1 for np.nan, even if np.nan is
  	# included in the index -> "repair" this here
  	if isna(rvalue).any() and isna(self.categories).any():
  	nan_pos = np.where(isna(self.categories))[0]
  	lindexer[lindexer == -1] = nan_pos

  (maybe?)

Maybe some others if people want to look.

xref #19787