coverage: Don't bother renumbering expressions on the Rust side

The LLVM API that we use to encode coverage mappings already has its own code
for removing unused coverage expressions and renumbering the rest.

This lets us get rid of our own complex renumbering code, making it easier to
change our coverage code in other ways.

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -479,7 +479,6 @@ TrivialTypeTraversalImpls! {
     ::rustc_target::asm::InlineAsmRegOrRegClass,
     crate::mir::coverage::CounterId,
     crate::mir::coverage::ExpressionId,
-    crate::mir::coverage::MappedExpressionIndex,
     crate::mir::Local,
     crate::mir::Promoted,
     crate::traits::Reveal,