nit: Drop mention of postgresql

Neutron is dropping all code related to it.

Change-Id: Ic29c3b463226d76c302fbd647bfd8c134a5fb434
diff --git a/neutron_tempest_plugin/common/constants.py b/neutron_tempest_plugin/common/constants.py
index f695f6c..36fb865 100644
--- a/neutron_tempest_plugin/common/constants.py
+++ b/neutron_tempest_plugin/common/constants.py
@@ -87,7 +87,7 @@
 
 ATTRIBUTES_TO_UPDATE = 'attributes_to_update'
 
-# Maximum value integer can take in MySQL and PostgreSQL
+# Maximum value integer can take in MySQL.
 # In SQLite integer can be stored in 1, 2, 3, 4, 6, or 8 bytes,
 # but here it will be limited by this value for consistency.
 DB_INTEGER_MAX_VALUE = 2 ** 31 - 1