Remove osmo-sgsn relics

* include/osmocom/sgsn -> gbproxy
* (unused) tests specific to osmo-sgsn

Change-Id: I3704760e8936bdd005a3f73727de5f2b3e8774a0
diff --git a/include/osmocom/Makefile.am b/include/osmocom/Makefile.am
index 09db97a..a3f6651 100644
--- a/include/osmocom/Makefile.am
+++ b/include/osmocom/Makefile.am
@@ -1,3 +1,3 @@
 SUBDIRS = \
-	sgsn \
+	gbproxy \
 	$(NULL)