Remove unused data file LogViewer api

Discovered in STORM-3133.

`findNMatches` in LogviewerLogSearchHandler returns a `Matched` object which contains a field `fileOffset`. However, in current implementation, `fileOffset` behaves a bit odd and is not being used anywhere in the app. I'm wondering if we should remove this field altogether

Specifically, the difference in behavior follows,
`fileOffset is passed in as the desired amount of file to skip in search (equiv to index of first file to search)

if desired amount of matches found, fileOffset will be the index of last scanned file (starting from 0).
if not enough matches found in all logs, fileOffset will be number of all logs (equiv to one past the index of last file)

See
https://github.com/apache/storm/pull/2754#discussion_r208691016
https://github.com/apache/storm/pull/2754#discussion_r208726809