tests: ui: compile-fail: pin_data: no_pin_on_phantompinned: fix warning

The `#[pin_data]` macro checks for fields with the type `PhantomPinned`
and then emits an error if it's not marked with `#[pin]`. This check
replaces the originally typed path with `::core::marker::PhantomPinned`
and thus any imports of the `PhantomPinned` stuff are not used. Opened
an issue: #51

Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Author: BennoLossin

tests/ui/compile-fail/pin_data/no_pin_on_phantompinned.rs

@@ -1,5 +1,5 @@
 use pin_init::*;
-use std::marker::{self, PhantomPinned};
+// use std::marker::{self, PhantomPinned};
 
 #[pin_data]
 struct Foo {

tests/ui/compile-fail/pin_data/no_pin_on_phantompinned.stderr

@@ -29,11 +29,3 @@ error: The field `pin4` of type `PhantomPinned` only has an effect, if it has th
   | ^^^^^^^^^^^
   |
   = note: this error originates in the macro `$crate::__pin_data` which comes from the expansion of the attribute macro `pin_data` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-warning: unused imports: `PhantomPinned` and `self`
- --> tests/ui/compile-fail/pin_data/no_pin_on_phantompinned.rs:2:19
-  |
-2 | use std::marker::{self, PhantomPinned};
-  |                   ^^^^  ^^^^^^^^^^^^^
-  |
-  = note: `#[warn(unused_imports)]` on by default