Delete some comment or messages of keystone_data.sh(removed)

As files/keystone_data.sh has been removed in the commit
https://review.openstack.org/#/c/79366/, we should remove some
related documations and comments.

Change-Id: I7802d0052fa28d8debb7f361d36a4f108869554c
diff --git a/lib/tempest b/lib/tempest
index 32630db..03e2aac 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -23,7 +23,7 @@
 #
 # Optional Dependencies:
 #
-# - ``ALT_*`` (similar vars exists in keystone_data.sh)
+# - ``ALT_*``
 # - ``LIVE_MIGRATION_AVAILABLE``
 # - ``USE_BLOCK_MIGRATION_FOR_LIVE_MIGRATION``
 # - ``DEFAULT_INSTANCE_TYPE``