Remove redundant MIR plugin structure

Redundant as of rust-lang/rust#40239.

This will be replaced instead with a late-stage lint pass (which is on
HIR, but has acces to type tables).

Author: OJFord

Cargo.toml

@@ -14,6 +14,3 @@ serde = "1.0"
 serde_derive = "1.0"
 serde_json = "1.0"
 tapioca-derive = {path="tapioca-derive"}
-
-[dependencies.tapioca_mir_plugin]
-path = "tapioca_mir_plugin"