remove unnecessary bound on Zip specialization impl

I originally added this bound in an attempt to make the specialization
sound for owning iterators but it was never correct here and the correct
and already implemented solution is is to place it on the IntoIter
implementation.

library/core/src/iter/adapters/zip.rs

@@ -429,13 +429,9 @@ where
     }
 }
 
+// Since SourceIter forwards the left hand side we do the same here
 #[unstable(issue = "none", feature = "inplace_iteration")]
-// Limited to Item: Copy since interaction between Zip's use of TrustedRandomAccess
-// and Drop implementation of the source is unclear.
-//
-// An additional method returning the number of times the source has been logically advanced
-// (without calling next()) would be needed to properly drop the remainder of the source.
-unsafe impl<A: InPlaceIterable, B: Iterator> InPlaceIterable for Zip<A, B> where A::Item: Copy {}
+unsafe impl<A: InPlaceIterable, B: Iterator> InPlaceIterable for Zip<A, B> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A: Debug, B: Debug> Debug for Zip<A, B> {