subchan: Remove unused const variable

nullbytes has never been used, I assume the code ended up
using memset instead of copying from nullbytes.

Fixes:
subchan_demux.c:47:22: warning: unused variable 'nullbytes' [-Wunused-const-variable]
static const uint8_t nullbytes[SYNC_HDR_BITS];
diff --git a/src/subchan_demux.c b/src/subchan_demux.c
index 4cd6094..f965490 100644
--- a/src/subchan_demux.c
+++ b/src/subchan_demux.c
@@ -44,7 +44,6 @@
 }
 
 #define SYNC_HDR_BITS	16
-static const uint8_t nullbytes[SYNC_HDR_BITS];
 
 /* check if we have just completed the 16 bit zero + 1 bit one sync
  * header, in accordance with GSM TS 08.60 Chapter 4.8.1 */