Drop alias `reduce` for `fold` - we have a `reduce` function

Searching for "reduce" currently puts the `reduce` alias for `fold`
above the actual `reduce` function. The `reduce` function already has a
cross-reference for `fold`, and vice versa.

library/core/src/iter/traits/iterator.rs

@@ -2133,7 +2133,6 @@ pub trait Iterator {
     /// ```
     ///
     /// [`reduce()`]: Iterator::reduce
-    #[doc(alias = "reduce")]
     #[doc(alias = "inject")]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]