Rollup merge of #152753 - cyrgani:remove-hack, r=petrochenkov

remove the explicit error for old `rental` versions

This was converted to a hard error 20 months ago (in rust-lang/rust#125596). This seems like enough time for anyone still using it to notice, so remove the note entirely now.
In comparison, the explicit note for the more impactful `time` breakage was already removed after 6 months (rust-lang/rust#129343).

Closes rust-lang/rust#73933.
Closes rust-lang/rust#83125.

r? @petrochenkov

Author: Zalathar