Refactor Array#[] and remove duplicated condition

Condition `length < 0` `ArrayNodes.IndexNode.slice` is also check a bit later in `ReadSliceNormalizedNode.readNegativeLength`.

Author: andrykonchin

src/main/java/org/truffleruby/core/array/ArrayNodes.java

@@ -305,9 +305,6 @@ protected Object indexFallback(RubyArray array, Object index, NotProvided length
         protected Object slice(RubyArray array, int start, int length,
                 @Cached ReadSliceNormalizedNode readSliceNode,
                 @Cached ConditionProfile negativeIndexProfile) {
-            if (length < 0) {
-                return nil;
-            }
             if (negativeIndexProfile.profile(start < 0)) {
                 start += array.size;
             }