core: now that .snapshot unit are gone, we don't need the per-type .no_gc bool anymore

src/core/unit.c

@@ -317,9 +317,6 @@ bool unit_check_gc(Unit *u) {
         if (state != UNIT_INACTIVE)
                 return true;
 
-        if (UNIT_VTABLE(u)->no_gc)
-                return true;
-
         if (u->no_gc)
                 return true;
 

src/core/unit.h

@@ -403,9 +403,6 @@ struct UnitVTable {
         /* Instances make no sense for this type */
         bool no_instances:1;
 
-        /* Exclude from automatic gc */
-        bool no_gc:1;
-
         /* True if transient units of this type are OK */
         bool can_transient:1;
 };