Drop extern declarations of global sgsn_instance in source files

There's an extern declaration already in sgsn.h.

Change-Id: I2375e13c8436a069e8fe60136c8e1759a9adc2d1

src/sgsn/gprs_gmm.c

@@ -67,8 +67,6 @@
 
 #define PTMSI_ALLOC
 
-extern struct sgsn_instance *sgsn;
-
 static const struct tlv_definition gsm48_gmm_att_tlvdef = {
 	.def = {
 		[GSM48_IE_GMM_CIPH_CKSN]	= { TLV_TYPE_FIXED, 1 },

src/sgsn/gprs_sgsn.c

@@ -61,7 +61,6 @@
 
 #define GPRS_LLME_CHECK_TICK 30
 
-extern struct sgsn_instance *sgsn;
 extern struct osmo_tdef sgsn_T_defs[];
 
 LLIST_HEAD(sgsn_mm_ctxts);

src/sgsn/sgsn_cdr.c

@@ -39,9 +39,6 @@
 #include <stdio.h>
 #include <inttypes.h>
 
-/* TODO...avoid going through a global */
-extern struct sgsn_instance *sgsn;
-
 /**
  * The CDR module will generate an entry like:
  *