vty: Use vty_install_default() instead of bsc_install_default()

Remove ournode_exit_cmd, ournode_end_cmd, and bsc_install_default()
since this functionality is provided by the current libosmocore.

Replace calls to bsc_install_default() by call to
vty_install_default() with the following semantic patch:

    @rule1@
    expression N;
    @@
    - bsc_install_default(N);
    + vty_install_default(N);

Ticket: OW#952
Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/sgsn_vty.c b/openbsc/src/gprs/sgsn_vty.c
index 8106b7d..5fb7145 100644
--- a/openbsc/src/gprs/sgsn_vty.c
+++ b/openbsc/src/gprs/sgsn_vty.c
@@ -418,7 +418,7 @@
 
 	install_element(CONFIG_NODE, &cfg_sgsn_cmd);
 	install_node(&sgsn_node, config_write_sgsn);
-	bsc_install_default(SGSN_NODE);
+	vty_install_default(SGSN_NODE);
 	install_element(SGSN_NODE, &cfg_sgsn_bind_addr_cmd);
 	install_element(SGSN_NODE, &cfg_ggsn_remote_ip_cmd);
 	//install_element(SGSN_NODE, &cfg_ggsn_remote_port_cmd);