Remove special case for ExprKind::Paren in MutVisitor

The special case breaks several useful invariants (`ExpnId`s are
globally unique, and never change). This special case
was added back in 2016 in https://github.com/rust-lang/rust/pull/34355

src/test/ui/lint/issue-87274-paren-parent.rs

@@ -0,0 +1,9 @@
+// check-pass
+// Tests that we properly lint at 'paren' expressions
+
+fn foo() -> Result<(), String>  {
+    (try!(Ok::<u8, String>(1))); //~ WARN use of deprecated macro `try`
+    Ok(())
+}
+
+fn main() {}

src/test/ui/lint/issue-87274-paren-parent.stderr

@@ -0,0 +1,10 @@
+warning: use of deprecated macro `try`: use the `?` operator instead
+  --> $DIR/issue-87274-paren-parent.rs:5:6
+   |
+LL |     (try!(Ok::<u8, String>(1)));
+   |      ^^^
+   |
+   = note: `#[warn(deprecated)]` on by default
+
+warning: 1 warning emitted
+