fix(nftables): always flush main table on start

On start created_tables will not contain the main "firewalld" table so a
flush command is not issued. We should always attempt to flush. If
CleanupOnExit=no, then not flushing causes duplicate rules on restart.

Fixes: rhbz2222044
(cherry picked from commit 6a155ea)

src/firewall/core/nftables.py

@@ -421,12 +421,10 @@ def build_flush_rules(self):
         self.policy_priority_counts = {}
         self.zone_source_index_cache = {}
 
-        rules = []
         if TABLE_NAME in self.created_tables["inet"]:
-            rules.append({"delete": {"table": {"family": "inet",
-                                               "name": TABLE_NAME}}})
             self.created_tables["inet"].remove(TABLE_NAME)
-        return rules
+
+        return self._build_delete_table_rules(TABLE_NAME)
 
     def _build_set_policy_rules_ct_rules(self, enable):
         add_del = { True: "add", False: "delete" }[enable]