Rollup merge of rust-lang#64896 - XAMPPRocky:remove-grammar, r=Centril

Remove legacy grammar

Revival of rust-lang#50835 & rust-lang#55545

On the #wg-grammar discord there was agreement that enough progress has been made to be able to remove the legacy grammar.

r? @Centril @qmx

cc @rust-lang/wg-grammar

Author: Centril