src: use include/osmocom/abis instead of include/osmocom/gsm/abis

Harald prefers short paths.

This patch also remove commented includes in several files.
diff --git a/src/subchan_demux.c b/src/subchan_demux.c
index 6ff66bc..73efd31 100644
--- a/src/subchan_demux.c
+++ b/src/subchan_demux.c
@@ -26,11 +26,9 @@
 #include <string.h>
 #include <errno.h>
 
-#include <osmocom/gsm/abis/subchan_demux.h>
-#include <osmocom/gsm/abis/trau_frame.h>
-//#include <openbsc/debug.h>
+#include <osmocom/abis/subchan_demux.h>
+#include <osmocom/abis/trau_frame.h>
 #include <talloc.h>
-//#include <openbsc/gsm_data.h>
 
 void *tall_tqe_ctx;