pkgtools: remove aria validation backend

Based on the community feedback, drop the aria validation backend
because it has been retired. Now we use opnfv-toscaparser as the
validation backend because it's used by various projects in ONAP and is
recommended by ONAP modeling sub-committee.

Issue-ID: VNFSDK-292
Change-Id: Iab65185b3303cad78a01ac930ce9e2760c5d4eb4
Signed-off-by: Lianhao Lu <lianhao.lu@intel.com>
diff --git a/requirements.txt b/requirements.txt
index e8df081..dbc2a26 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,5 @@
 ruamel.yaml<0.15
 requests>=2.3.0
-#apache-ariatosca==0.1.1
-stevedore >= 1.9.0
+stevedore>=1.9.0
 udatetime<1.0,>=0.0.16
 nfv-toscaparser<2.0,>=1.0.1