Remove #![warn(unreachable_pub)] from all compiler/ crates.

It's no longer necessary now that `-Wunreachable_pub` is being passed.

compiler/rustc_session/src/lib.rs

@@ -7,7 +7,6 @@
 // To generate CodegenOptionsTargetModifiers and UnstableOptionsTargetModifiers enums
 // with macro_rules, it is necessary to use recursive mechanic ("Incremental TT Munchers").
 #![recursion_limit = "256"]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub mod errors;