Auto merge of #25157 - alexcrichton:remove-vec-add, r=aturon

Ideally this trait implementation would be unstable, requiring crates to opt-in
if they would like the functionality, but that's not currently how stability
works so the implementation needs to be removed entirely.

This may come back at a future date, but for now the conservative option is to
remove it.

[breaking-change]

src/libcollections/vec.rs

@@ -67,7 +67,7 @@ use core::intrinsics::assume;
 use core::iter::{repeat, FromIterator};
 use core::marker::PhantomData;
 use core::mem;
-use core::ops::{Index, IndexMut, Deref, Add};
+use core::ops::{Index, IndexMut, Deref};
 use core::ops;
 use core::ptr;
 use core::ptr::Unique;
@@ -1622,17 +1622,6 @@ impl<T: Ord> Ord for Vec<T> {
     }
 }
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T: Clone> Add<&'a [T]> for Vec<T> {
-    type Output = Vec<T>;
-
-    #[inline]
-    fn add(mut self, rhs: &[T]) -> Vec<T> {
-        self.push_all(rhs);
-        self
-    }
-}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Drop for Vec<T> {
     fn drop(&mut self) {