Removed a fixme leftover.

We have a proper version in stdlib now.

runtime/src/main/kotlin/kotlin/collections/Collections.kt

@@ -298,14 +298,6 @@ fun <E> Array<E>.toSet(): Set<E> {
     return result
 }
 
-@FixmeVariance
-public fun <T, C : MutableCollection</*in */T>> Iterable<T>.toCollection(destination: C): C {
-    for (item in this) {
-        destination.add(item)
-    }
-    return destination
-}
-
 @Fixme
 internal fun <T> List<T>.optimizeReadOnlyList() = this
 
@@ -2430,4 +2422,4 @@ public fun Iterable<Double>.sum(): Double {
         sum += element
     }
     return sum
-}
+}