Revert "Add rustfmt::skip as a work around"

This reverts commit 0e80341.

Fixed by rust-lang/rustfmt#4528.

Author: giraffate

tests/ui/cast_ref_to_mut.rs

@@ -2,10 +2,6 @@
 #![allow(clippy::no_effect)]
 
 extern "C" {
-    #[rustfmt::skip]
-    // TODO: This `rustfmt::skip` is a work around of #6336 because
-    // the following comments are checked by rustfmt for some reason.
-    //
     // N.B., mutability can be easily incorrect in FFI calls -- as
     // in C, the default is mutable pointers.
     fn ffi(c: *mut u8);

tests/ui/cast_ref_to_mut.stderr

@@ -1,19 +1,19 @@
 error: casting `&T` to `&mut T` may cause undefined behavior, consider instead using an `UnsafeCell`
-  --> $DIR/cast_ref_to_mut.rs:22:9
+  --> $DIR/cast_ref_to_mut.rs:18:9
    |
 LL |         (*(a as *const _ as *mut String)).push_str(" world");
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::cast-ref-to-mut` implied by `-D warnings`
 
 error: casting `&T` to `&mut T` may cause undefined behavior, consider instead using an `UnsafeCell`
-  --> $DIR/cast_ref_to_mut.rs:23:9
+  --> $DIR/cast_ref_to_mut.rs:19:9
    |
 LL |         *(a as *const _ as *mut _) = String::from("Replaced");
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: casting `&T` to `&mut T` may cause undefined behavior, consider instead using an `UnsafeCell`
-  --> $DIR/cast_ref_to_mut.rs:24:9
+  --> $DIR/cast_ref_to_mut.rs:20:9
    |
 LL |         *(a as *const _ as *mut String) += " world";
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^