Rollup merge of rust-lang#25081 - carols10cents:remove-extract-grammar, r=steveklabnik

 This script used to be used to [extract the grammar sections from the reference](rust-lang#8585), but there is [now a separate src/doc/grammar.md](rust-lang#22308) that generates grammar.html where the grammar sections that used to be in the reference live, so there is no longer a need to extract the grammar from the reference.

I ❤️ deleting code :) But I totally understand if there's a reason to keep this around that I don't know about :)

Author: Manishearth