DOC: Remove mention of NaN and NAN aliases from constants

Remove aliases from docs, they don't exist anymore.

Closes numpygh-26732

Author: seberg

doc/source/reference/constants.rst

@@ -88,8 +88,6 @@ NumPy includes several constants:
     NumPy uses the IEEE Standard for Binary Floating-Point for Arithmetic
     (IEEE 754). This means that Not a Number is not equivalent to infinity.
 
-    `NaN` and `NAN` are aliases of `nan`.
-
     .. rubric:: Examples
 
     >>> np.nan