Remove time-boxed `time` inference regression mitigation note

[jieyouxu]

We should remove the time inference regression diagnostics note once sufficient time has passed.

Introduced in #129343 to help guide users to update their time version.

See #129343 (comment) for more context.

cc @apiraino as you wanted to make sure it gets removed eventually (thank you!)

[apiraino]

Reminder for myself, Esteban set a timebomb ⏲️ for Rust 1.89, that should be enough to keep track of this (also see your comment).

[jieyouxu]

(#133245 in case we forgor)