Remove the unused isStream property on various Streams

This property was added all the way back in PR 542, but hasn't actually been relied upon ever since PR 692.
Note that there's a `isStream()` utility function which replaced the property years ago, hence the `isStream` property is now dead code.

src/core/chunked_stream.js

@@ -271,8 +271,6 @@ var ChunkedStream = (function ChunkedStreamClosure() {
       subStream.dict = dict;
       return subStream;
     },
-
-    isStream: true
   };
 
   return ChunkedStream;

src/core/stream.js

@@ -129,7 +129,6 @@ var Stream = (function StreamClosure() {
     makeSubStream: function Stream_makeSubStream(start, length, dict) {
       return new Stream(this.bytes.buffer, start, length, dict);
     },
-    isStream: true
   };
 
   return Stream;