gh-38835: Remove redundant modulo operation in vector_modn_dense
Just a tiny change. There's no user-visible behavior seen.
(The output of `int(…)` is already reduced modulo `p` anyway.)
### :memo: Checklist
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preview.
URL: https://github.com/sagemath/sage/pull/38835
Reported by: user202729
Reviewer(s): Frédéric Chapoton, Travis Scrimshaw, user202729
