Remove unnecessary lifetime argument from arena macros.

Because it's always `'tcx`. In fact, some of them use a mixture of
passed-in `$tcx` and hard-coded `'tcx`, so no other lifetime would even
work.

This makes the code easier to read.

compiler/rustc_ast_lowering/src/lib.rs

@@ -84,7 +84,7 @@ mod item;
 mod pat;
 mod path;
 
-rustc_hir::arena_types!(rustc_arena::declare_arena, 'tcx);
+rustc_hir::arena_types!(rustc_arena::declare_arena);
 
 struct LoweringContext<'a, 'hir: 'a> {
     /// Used to assign IDs to HIR nodes that do not directly correspond to AST nodes.