[Z3py] Consistent behavior of eq and ne for FP

Before, x == y and x != y were returning inconsistent expressions (i.e.
`Not(x == y)` was not the same as `x != y`):

>>> x = FP('x', Float32())
>>> y = FP('y', Float32())
>>> (x == y).sexpr()
'(= x y)'
>>> (x != y).sexpr()
'(not (fp.eq x y))'

`=` does not have the same semantics as `fp.eq` (e.g. `fp.eq` of +0.0
and -0.0 is true while it is false for `=`).
This patch removes the __ne__ method from FPRef, so `x == y` and `x !=
y` use the inherited operations while fpEQ and fpNEQ can be used to
refer to `fp.eq(..)`/`Not(fp.eq(..))`.

src/api/python/z3.py

@@ -8099,10 +8099,6 @@ class FPRef(ExprRef):
     def __gt__(self, other):
         return fpGT(self, other, self.ctx)
 
-    def __ne__(self, other):
-        return fpNEQ(self, other, self.ctx)
-
-
     def __add__(self, other):
         """Create the Z3 expression `self + other`.