gbproxy: Add a command to delete peers from the GBProxy

This just deletes the peer entry based on NSEI and BVCI. The NS-VC
are not touched.
diff --git a/openbsc/src/gprs/gb_proxy_vty.c b/openbsc/src/gprs/gb_proxy_vty.c
index 63546d3..176ea65 100644
--- a/openbsc/src/gprs/gb_proxy_vty.c
+++ b/openbsc/src/gprs/gb_proxy_vty.c
@@ -80,6 +80,8 @@
 {
 	install_element_ve(&show_gbproxy_cmd);
 
+	install_element(ENABLE_NODE, &delete_gb_cmd);
+
 	install_element(CONFIG_NODE, &cfg_gbproxy_cmd);
 	install_node(&gbproxy_node, config_write_gbproxy);
 	bsc_install_default(GBPROXY_NODE);