Added method remove_torrents to core.

Removing multiple torrents with remove_torrents will be faster
than calling remove_torrent for each torrent, as the state file
will be written only once, after all the torrents are deleted.
Thats not mine actually ;-)
https://github.com/bendikro/deluge/blame/83df1159c096260f233c5b0bd162058b7b7990e7/deluge/core/torrentmanager.py#L620