Removed unused variables in rfc2863.py (sonic-net#237)

**- What I did**

Removed unused variables rif_port_map and port_rif_map in rfc2863.py

**- How I did it**

Edited rfc2863.py as requested by @qiluo-msft in merged PR sonic-net#218.

**- How to verify it**

pytest-3 test_interfaces.py and pytest-3 namespace/test_interfaces.py in sonic-snmpagent/tests.

**- Description for the changelog**

Removed unused variables.

(cherry picked from commit a07da53)

src/sonic_ax_impl/mibs/ietf/rfc2863.py

@@ -93,7 +93,6 @@ def __init__(self):
         self.mgmt_alias_map = {}
         self.vlan_oid_name_map = {}
         self.vlan_name_map = {}
-        self.rif_port_map = {}
         self.if_counters = {}
         self.if_range = []
         self.if_name_map = {}
@@ -128,9 +127,6 @@ def reinit_data(self):
         self.vlan_oid_sai_map, \
         self.vlan_oid_name_map = Namespace.get_sync_d_from_all_namespace(mibs.init_sync_d_vlan_tables, self.db_conn)
 
-        self.rif_port_map, \
-        self.port_rif_map = Namespace.get_sync_d_from_all_namespace(mibs.init_sync_d_rif_tables, self.db_conn)
-
         self.if_range = sorted(list(self.oid_name_map.keys()) +
                                list(self.oid_lag_name_map.keys()) +
                                list(self.mgmt_oid_name_map.keys()) +