Migrate flush_unused_database from py-redis to sonic-swss-common (#15511)

Migrate flush_unused_database from py-redis to sonic-swss-common

#### Why I did it
flush_unused_database using py-redis, but sonic-swss-common already support flushdb, so we need migrate to sonic-swss-common

##### Work item tracking
- Microsoft ADO **(number only)**: 24292565

#### How I did it
Migrate flush_unused_database from py-redis to sonic-swss-common

#### How to verify it
Pass all UT and E2E test

#### Description for the changelog
Migrate flush_unused_database from py-redis to sonic-swss-common

Author: liuh-80

dockers/docker-database/flush_unused_database

@@ -1,6 +1,5 @@
 #!/usr/bin/python3
 from swsscommon import swsscommon
-import redis
 import subprocess
 import time
 import syslog
@@ -26,7 +25,18 @@ for instname, v in instlists.items():
             continue
 
         try:
-            r = redis.Redis(host=insthost, unix_socket_path=instsocket, db=dbid)
-            r.flushdb()
-        except (redis.exceptions.ConnectionError):
+            # Migrate code from py-redis to swsscommon, original code:
+            #    r = redis.Redis(host=insthost, unix_socket_path=instsocket, db=dbid)
+            #    py-redis will use TCP connection when unix_socket_path is None
+            #    https://github.com/redis/redis-py/blob/d95d8a24ed2af3eae80b7b0f14cbccc9dbe86e96/redis/client.py#L1006
+            if instsocket is not None:
+                # connect with Unix socket
+                connector = swsscommon.DBConnector(dbid, instsocket, 0)
+            else:
+                # connect with TCP socket
+                port = swsscommon.SonicDBConfig.getDbPort(dbname);
+                connector = swsscommon.DBConnector(dbid, insthost, port, 0)
+
+            connector.flushdb()
+        except RuntimeError:
            syslog.syslog(syslog.LOG_INFO,"flushdb:Redis Unix Socket connection error for path {} and dbaname {}".format(instsocket, dbname))