Bug 33838: Don't reset the renewal count

Bug 33838: Don't reset the renewal count

I cannot find any justification for this line existing.
MarkIssueReturned does not do this.

Resetting item's renewal count was introduced in bug 5877 with no explanation.

Test Plan:
1) Check out item 3999900000001 to a patron
2) Upload the KOC file attached to this bug report
3) Navigate the Pending Offline Circ actions, see your return is listed
4) Click the checkbox to select your return
5) Click Process once. Nothing appears to happen
6) In a new tab, pull up the bib for item 3999900000001, see that it has been checked in
7) Back on your Offline Circ tab, click Process a second time
8) Koha tells you the item is not checked out
9) Apply this patch, restart all the things!
10) Repeat steps 1-4, everything should now work!

Signed-off-by: Andrew Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Edit: this line was introduced by bug 30275, by mistake. The
renewals_count attribute belongs to the `issues` table, and I agree it
shouldn't be set to 0 at all as it will (with no reason) make us loose
the value!
Tests pass with and without this change, so this isn't even tested.
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>