enable missing_const_for_thread_local lint, now checks for MSRV (see rust-lang/rust-clippy#12404)

Author: hkratz

futures-executor/src/lib.rs

@@ -46,7 +46,6 @@
 ))]
 #![warn(missing_docs, unsafe_op_in_unsafe_fn)]
 #![cfg_attr(docsrs, feature(doc_cfg))]
-#![allow(clippy::missing_const_for_thread_local)] // clippy bug: this lint doesn't consider MSRV
 
 #[cfg(feature = "std")]
 extern crate std;