Get rid of the NSCache for cells - UITableView knows how to cache cells

Blanks rows were appearing erratically while scrolling around a notification comment thread.
Get rid of the NSCache for cells - UITableView knows how to cache cells

Blanks rows were appearing erratically while scrolling around a notification comment thread.
Merge pull request #968 from wordpress-mobile/issue/954-cell-reuse-note-comments

Notifications comments blanking around during scrolling