Remove RawVec::cap()

As suggested in https://github.com/rust-lang/rust/pull/60340#issuecomment-493681032

src/liballoc/raw_vec.rs

@@ -209,12 +209,6 @@ impl<T, A: Alloc> RawVec<T, A> {
         }
     }
 
-    // For backwards compatibility
-    #[inline(always)]
-    pub fn cap(&self) -> usize {
-        self.capacity()
-    }
-
     /// Returns a shared reference to the allocator backing this RawVec.
     pub fn alloc(&self) -> &A {
         &self.a