gh-37792: remove a stray leftover in in sa2si_ZZmod()
    
As reported on [sage-devel](https://groups.google.com/g/sage-
devel/c/r7IpxjZTz5Y/m/d0I95VSnAAAJ) and reproduced, the following
crashes Sage with probability about 1/3.
```python
x,y=var('x,y');n=10**6
while True: so=solve_mod(x*y-1,n)
#press CTL-C, crashes with probability about 1/3
```
The backtrace points to is a meaningless assignment in `sa2si_ZZmod()`
```python
nr2mModul = d.parent().characteristic()
```
where this happens.  `nr2mModul` is not used anywhere in the code.
Removing it appears to make the crash to go away.




### 📝 Checklist

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- [x] The title is concise and informative.
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URL: #37792
Reported by: Dima Pasechnik
Reviewer(s):

Author: Release Manager

src/sage/libs/singular/singular.pyx

@@ -1544,7 +1544,6 @@ cdef inline number *sa2si_ZZmod(IntegerMod_abstract d, ring *_ring) noexcept:
         sage: P(3)
         3
     """
-    nr2mModul = d.parent().characteristic()
     if _ring != currRing: rChangeCurrRing(_ring)
 
     cdef number *nn