lib:sha1: remove unused constant

This prevents a clang warning.
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>

lib/sha1.c

@@ -389,8 +389,6 @@ void sha1_hmac(const unsigned char *key, int keylen,
 	memset (&ctx, 0, sizeof (sha1_context));
 }
 
-static const char _sha1_src[] = "_sha1_src";
-
 #ifdef SELF_TEST
 /*
  * FIPS-180-1 test vectors