Delete special handling of some expr kinds from print_let

In all four of Break, Closure, Ret, Yeet, the needs_par_as_let_scrutinee
is guaranteed to return true because the .precedence().order() of those
expr kinds is <= AssocOp::LAnd.precedence().

The relevant functions in rustc_ast::util::parser are:

    fn needs_par_as_let_scrutinee(order: i8) -> bool {
        order <= prec_let_scrutinee_needs_par() as i8
    }

    fn prec_let_scrutinee_needs_par() -> usize {
        AssocOp::LAnd.precedence()
    }

The .precedence().order() of Closure is PREC_CLOSURE (-40) and of Break,
Ret, Yeet is PREC_JUMP (-30).

The value of AssocOp::LAnd.precedence() is 6.

So this commit causes no change in behavior, only potentially
performance by doing a redundant call to contains_exterior_struct_lit in
those four cases. This is fine because Break, Closure, Ret, Yeet should
be exceedingly rare in the position of a let scrutinee.

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1161,16 +1161,8 @@ impl<'a> State<'a> {
         self.word_space("=");
         self.print_expr_cond_paren(
             expr,
-            match expr.kind {
-                ast::ExprKind::Break(..)
-                | ast::ExprKind::Closure(..)
-                | ast::ExprKind::Ret(..)
-                | ast::ExprKind::Yeet(..) => true,
-                _ => {
-                    parser::contains_exterior_struct_lit(expr)
-                        || parser::needs_par_as_let_scrutinee(expr.precedence().order())
-                }
-            },
+            parser::contains_exterior_struct_lit(expr)
+                || parser::needs_par_as_let_scrutinee(expr.precedence().order()),
         );
     }