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.
### :memo: Checklist
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URL: https://github.com/sagemath/sage/pull/37792
Reported by: Dima Pasechnik
Reviewer(s):
