ospfd: fix internal ldp-sync state flags when feature is disabled

ospfd: fix internal ldp-sync state flags when feature is disabled

When enabling "mpls ldp-sync" under "router ospf" ospfd configures
SET_FLAG(ldp_sync_info->flags, LDP_SYNC_FLAG_IF_CONFIG) so internally knowing
that the ldp-sync feature is enabled. However the flag is not cleared when
turning of the feature using "nompls ldp-sync"!

https://github.com/FRRouting/frr/issues/16375

Signed-off-by: Christian Breunig <christian@breunig.cc>
(cherry picked from commit 5a70378a47f541b0354fbb96770dd0a65ec552b8)