Fix #0016933: Deleting relationship should set target bug's last updated

1) bug_relationship_delete changed to remove comment about updating date
for just the source bug
2) bug_relationship_delete changed to add bug_update_date for dest bug.

Note: step 2 above matches the behaviour already within the SOAP api,
which updates both bug timestamps (as per 16933  and related 12628)

3) add bug_update_date to duplicate bug id in bug_update

Marking a bug as duplicate did not update bug update timestamp of both bugs

We've previously argued that adding/removing a relationship should trigger
a timestamp update

4) add bug_update_date to similar logic as 3 in bug_api.php

Note: Related Reference issues for this in bugtracker are:

0016933: Deleting relationship should set target bug's last updated
0012628: Update last change date for both bugs when updating a relationship
0014417: Adding duplicate relationship should not set "modified" date of parent issue
0007453: Difference between last_updated in bug_table and date_modified in date_modified
0008911: adding relationship does only change field "last changed" of one issue