Remove unused python-etcd3

python-etcd3 can't be used since etcd3 driver was removed from tooz in
tooz 5.0.0 [1].

[1] 6bc02cda5b452bbf4821621eafc031bd676f8a2f

Change-Id: I30b895b4473e2676085e27969a43b91be9b1e413

lib/libraries

@@ -131,12 +131,7 @@ function install_libs {
     # python client libraries we might need from git can go here
     _install_lib_from_source "python-barbicanclient"
 
-
     # etcd (because tooz does not have a hard dependency on these)
-    #
-    # NOTE(sdague): this is currently a work around because tooz
-    # doesn't pull in etcd3.
-    pip_install etcd3
     pip_install etcd3gw
 }