tvheadend: Remove empty service_preupgrade causing error

On DSM6 the empty service_preupgrade() was causing:
2021/06/19 06:24:46	upgrade tvheadend 4.3.20210612-27 Begin postuninst
/var/packages/tvheadend/scripts/service-setup: line 70: syntax error near unexpected token `}'
/var/packages/tvheadend/scripts/service-setup: line 70: `}
Removing it solved the issue

spk/tvheadend/src/service-setup.sh

@@ -40,10 +40,6 @@ service_postinst ()
              ${WHEELHOUSE} ${WHEELHOUSE}/*.whl
 }
 
-service_preupgrade ()
-{
-}
-
 service_postupgrade ()
 {
     # Need to enforce correct permissions for recording directories on upgrades