This PR replaces the proof of the simplification lemma `Nat.zero_mod`
with
`rfl` since it is, by design, a definitional equality. This solves an
issue
whereby the lemma could not be used by the simplifier when in 'dsimp'
mode.
Closes#9389
---------
Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
