Fix for gpcheckcat when issue gpcheckcat -C pg_class (#16902)

Error occurs when we issue command `gpcheckcat -C pg_class`.
Reported error is "[ERROR] executing: Cross consistency check for pg_class\n  Execution error: name 'db' is not defined".
This is because of use of an undefined variable 'db'.
This commit fixes the issue by removing its usage.

Authored-by: vrhappy <songlong88@126.com>

Author: vrhappy

gpMgmt/bin/gpcheckcat

@@ -1255,7 +1255,7 @@ def checkTableACL(cat):
     try:
         conn = connect2(GV.cfg[GV.coordinator_dbid], utilityMode=False)
         with conn.cursor() as curs:
-            curs = db.execute(qry)
+            curs.execute(qry)
             nrows = curs.rowcount
 
             if nrows == 0:

gpMgmt/test/behave/mgmt_utils/gpcheckcat.feature

@@ -9,6 +9,9 @@ Feature: gpcheckcat tests
         Given database "all_good" is dropped and recreated
         Then the user runs "gpcheckcat -A"
         Then gpcheckcat should return a return code of 0
+        When the user runs "gpcheckcat -C pg_class"
+        Then gpcheckcat should return a return code of 0
+        And gpcheckcat should not print "Execution error:" to stdout
         And the user runs "dropdb all_good"
 
     Scenario: gpcheckcat should drop leaked schemas