discover: Don't free URL in load_url

Previously, load_url took a char * argument, from which it parsed a
newly allocated URL, and freed the URL before returning.

Commit 5be946c changed load_url (then load_file) to accept a parsed URL
instead of a char *, but didn't remove the free. Any URLs passed to
load_url are currently being unintionally free()ed.

This change removes the invalid free.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>