miner-fs: Fix tracking file move

Identified by MinerCrawlTest::test_07_move_from_monitored_to_monitored
(300-miner-basic-ops.py)

In item_move() it fails to get source_iri, does not check it's validity and
uses it in the DELETE expression of the SPARQL query constructed there.

Broken since d836f00 (libtracker-miner: Store iri transiently as GFile
qdata) - tracker_file_notifier_get_file_iri() is added 'force' argument
and (wrapped with lookup_file_urn) passed force=FALSE from item_move().
This call then fails for regular files because only directories are
cached once crawling has completed as stated in the comment in
finish_current_directory() in libtracker-miner/tracker-file-notifier.c.

https://bugzilla.gnome.org/show_bug.cgi?id=678986
miner-fs: Remove unused argument is_new

https://bugzilla.gnome.org/show_bug.cgi?id=678986