Drop unused environment

The GET_PIP_CACHE_LOCATION environment is used in octavia to use
cache fo pip during disk image build, but is not at all used in
ovn-octavia-provider itself.

Change-Id: Id7fbbdbf0e02a0dbaf16685194a62e76d5ab4c91

Author: kajinamit

devstack/plugin.sh

@@ -1,8 +1,5 @@
 #!/usr/bin/env bash
 
-# devstack plugin for octavia
-GET_PIP_CACHE_LOCATION=/opt/stack/cache/files/get-pip.py
-
 # How to connect to ovsdb-server hosting the OVN NB database
 if is_service_enabled tls-proxy; then
    OVN_PROTO=ssl