modules: move 'content' and 'anchors' to MODULE_COMMON_ATTRS

All modules implement these. The only one that differs is the i3
module. It still implements them, but 'content' uses an internal
verifier function.

modules/i3.c

@@ -651,7 +651,7 @@ verify_conf(keychain_t *chain, const struct yml_node *node)
         {"right-spacing", false, &conf_verify_int},
         {"content", true, &verify_content},
         {"anchors", false, NULL},
-        MODULE_COMMON_ATTRS,
+        {NULL, false, NULL},
     };
 
     return conf_verify_dict(chain, node, attrs);