coverage: CodeRegion is never stored in an arena

This might have been left over when coverage regions were stored in individual
MIR statements, instead of a separate table attached to the MIR body.

compiler/rustc_middle/src/arena.rs

@@ -35,7 +35,6 @@ macro_rules! arena_types {
             )>,
             [] crate_for_resolver: rustc_data_structures::steal::Steal<(rustc_ast::Crate, rustc_ast::AttrVec)>,
             [] resolutions: rustc_middle::ty::ResolverGlobalCtxt,
-            [decode] code_region: rustc_middle::mir::coverage::CodeRegion,
             [] const_allocs: rustc_middle::mir::interpret::Allocation,
             [] region_scope_tree: rustc_middle::middle::region::ScopeTree,
             // Required for the incremental on-disk cache

compiler/rustc_middle/src/ty/codec.rs

@@ -462,7 +462,6 @@ impl_decodable_via_ref! {
     &'tcx traits::ImplSource<'tcx, ()>,
     &'tcx mir::Body<'tcx>,
     &'tcx mir::BorrowCheckResult<'tcx>,
-    &'tcx mir::coverage::CodeRegion,
     &'tcx ty::List<ty::BoundVariableKind>,
     &'tcx ty::ListWithCachedTypeInfo<ty::Clause<'tcx>>,
     &'tcx ty::List<FieldIdx>,