[VOL-3107] Add DeleteGroup API to OpenOLT Agent

- DeleteGroup proto API method implemented.
- Unit tests added.

Change-Id: If196a92f7a717fc4cc6c2f0e0bb6ca74fc3683d2
diff --git a/protos/Makefile b/protos/Makefile
index 376ea8c..652329b 100644
--- a/protos/Makefile
+++ b/protos/Makefile
@@ -19,7 +19,7 @@
 # This specifies the GIT tag in https://github.com/opencord/voltha-protos
 # repo that we need to refer to, to pick the right version of
 # openolt.proto and tech_profile.proto
-OPENOLT_PROTO_VER ?= v3.3.6
+OPENOLT_PROTO_VER ?= v3.3.9
 GRPC_VER ?= v1.10.x
 
 CXX ?= g++