trie: small optimization of delete in fullNode case

[Evolution404]

when nn is not nil, there is no need to check the number of non-nil entries.
Since n must've contained at least two children before deletion, there must be another child node other than nn.

[holiman]

I see you opened a separate PR instead of #22960. Next time, just make another commit on top. We squash the commits into one when merging anyway.
If you want, you can squash it yourself

git commit -m "foo bar"
git rebase -i HEAD~2
# Then select the latest for squashing into the previous
# and finally, since it's squashed, you need to force-push
git push -f 

[Evolution404]

Thanks for reminding

[fjl]

Very nice!