qdev: Delete unused LostTickPolicy "merge"

Commit 4e4fa398 "qdev: Introduce lost tick policy property"
(v1.1.0) created PropertyType PROP_TYPE_LOSTTICKPOLICY with values
"discard", "delay", "merge", and "slew".  Value "merge" has never been
used.  Delete it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190401150140.29151-1-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>