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

src/asn1helpers.c

@@ -65,9 +65,6 @@ int BIT_STRING_fromBuf(BIT_STRING_t *st, const uint8_t *str, unsigned int bit_le
 		return 0;
 	}
 
-	if (len < 0)
-		len = strlen((char*)str);
-
 	buf = MALLOC(len);
 	if (!buf) {
 		errno = ENOMEM;