migrations: Remove pgroonga check for postgres database.

Zulip has only supported postgres for a long time, and this is
unlikely to change, so this code was just extra clutter.

pgroonga/migrations/0001_enable.py

@@ -9,7 +9,7 @@ class Migration(migrations.Migration):
     ]
 
     database_setting = settings.DATABASES["default"]
-    if "postgres" in database_setting["ENGINE"]:
+
     operations = [
         migrations.RunSQL(
             [
@@ -51,5 +51,3 @@ END$$
             ],
         ),
     ]
-    else:
-        operations = []