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

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

compiler/rustc_mir_dataflow/src/lib.rs

@@ -7,7 +7,6 @@
 #![feature(let_chains)]
 #![feature(never_type)]
 #![feature(try_blocks)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use rustc_middle::ty;