Remove unused extern crate rustc_symbol_mangling

Author: bjorn3

compiler/rustc_codegen_gcc/src/lib.rs

@@ -20,7 +20,6 @@ extern crate rustc_metadata;
 extern crate rustc_middle;
 extern crate rustc_session;
 extern crate rustc_span;
-extern crate rustc_symbol_mangling;
 extern crate rustc_target;
 
 // This prevents duplicating functions and statics that are already part of the host rustc process.