fix compiler warning: drop dead code from BIT_STRING_fromBuf()

The warning is, on FreeBSD,

  CC       asn1helpers.lo
asn1helpers.c:68:10: error: comparison of unsigned expression < 0 is always false [-Werror,-Wtautological-compare]
        if (len < 0)
            ~~~ ^ ~

Change-Id: I80867da697d744d7ef4d70c8f24031f5781fb11a
diff --git a/src/asn1helpers.c b/src/asn1helpers.c
index a4ced70..4e5e08d 100644
--- a/src/asn1helpers.c
+++ b/src/asn1helpers.c
@@ -65,9 +65,6 @@
 		return 0;
 	}
 
-	if (len < 0)
-		len = strlen((char*)str);
-
 	buf = MALLOC(len);
 	if (!buf) {
 		errno = ENOMEM;