[improper_ctypes] Don't suggest raw pointers when encountering trait objects

It's unhelpful since raw pointers to trait objects are also FFI-unsafe and casting to a thin raw pointer loses the vtable. There are working solutions that _involve_ raw pointers but they're too complex to explain in one line and have serious trade offs.

src/librustc_lint/types.rs

@@ -577,7 +577,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             ty::TyDynamic(..) => FfiUnsafe {
                 ty: ty,
                 reason: "trait objects have no C equivalent",
-                help: Some("consider using a raw pointer instead"),
+                help: None,
             },
 
             ty::TyStr => FfiUnsafe {

src/test/ui/lint-ctypes.stderr

@@ -78,8 +78,6 @@ error: `extern` block uses type `std::clone::Clone` which is not FFI-safe: trait
    |
 62 |     pub fn trait_type(p: &Clone); //~ ERROR uses type `std::clone::Clone`
    |                          ^^^^^^
-   |
-   = help: consider using a raw pointer instead
 
 error: `extern` block uses type `(i32, i32)` which is not FFI-safe: tuples have unspecified layout
   --> $DIR/lint-ctypes.rs:63:26