Remove jobserver from Session

It is effectively a global resource and the jobserver::Client in Session
was a clone of GLOBAL_CLIENT anyway.

compiler/rustc_codegen_cranelift/src/lib.rs

@@ -12,7 +12,6 @@
 #![warn(unused_lifetimes)]
 // tidy-alphabetical-end
 
-extern crate jobserver;
 #[macro_use]
 extern crate rustc_middle;
 extern crate rustc_abi;