lib:sha1: remove unused constant

This prevents a clang warning.
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
diff --git a/lib/sha1.c b/lib/sha1.c
index 0a5f688..05b17a2 100644
--- a/lib/sha1.c
+++ b/lib/sha1.c
@@ -389,8 +389,6 @@
 	memset (&ctx, 0, sizeof (sha1_context));
 }
 
-static const char _sha1_src[] = "_sha1_src";
-
 #ifdef SELF_TEST
 /*
  * FIPS-180-1 test vectors