Remove duplicate nullity check line (ReactiveX#5723)

Author: philleonard

src/main/java/io/reactivex/Observable.java

@@ -12538,7 +12538,6 @@ public final <K, V> Single<Map<K, V>> toMap(
             final Function<? super T, ? extends V> valueSelector,
             Callable<? extends Map<K, V>> mapSupplier) {
         ObjectHelper.requireNonNull(keySelector, "keySelector is null");
-        ObjectHelper.requireNonNull(keySelector, "keySelector is null");
         ObjectHelper.requireNonNull(valueSelector, "valueSelector is null");
         ObjectHelper.requireNonNull(mapSupplier, "mapSupplier is null");
         return collect(mapSupplier, Functions.toMapKeyValueSelector(keySelector, valueSelector));