Rollup merge of #106714 - Ezrashaw:remove-e0490, r=davidtwco

remove unreachable error code `E0490`

AFAIK, the untested and undocumented error code `E0490` is now unreachable, it was from the days of the original borrow checker.

cc ``@GuillaumeGomez`` #61137

Author: Noratrieb