Both FragmentedMp4Extractor and WebmExtractor now invoke seekMap() with
SeekMap.UNSEEKABLE if there isn't an index in the stream, so DASH playbacks
will end up printing this warning every 5 seconds or so. This is obviously
not desirable, so this CL just removes the warning! The alternative would
have been to print a warning if the value is anything other than UNSEEKABLE,
but it doesn't really seem worth it.
