Remove osmo-sgsn relics

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

Change-Id: I3704760e8936bdd005a3f73727de5f2b3e8774a0
diff --git a/configure.ac b/configure.ac
index a4758b5..309d3f7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -236,7 +236,7 @@
 AC_OUTPUT(
     include/Makefile
     include/osmocom/Makefile
-    include/osmocom/sgsn/Makefile
+    include/osmocom/gbproxy/Makefile
     src/Makefile
     tests/Makefile
     doc/Makefile