Remove out-of-date comment.

Exhaustiveness and usefulness checking are now in
`rustc_pattern_analysis`.

Author: nnethercote

compiler/rustc_mir_build/src/lib.rs

@@ -1,6 +1,4 @@
 //! Construction of MIR from HIR.
-//!
-//! This crate also contains the match exhaustiveness and usefulness checking.
 
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]