builtin-tag.c: remove cruft

After changing builtin-tag.c to use strbuf in fd17f5b5 (Replace all
read_fd use with strbuf_read, and get rid of it.), the last condition
in do_sign() will always be false, as it's checked already right
above.  So let's remove the cruft.
Signed-off-by: NGerrit Pape <pape@smarden.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-tag.c

@@ -236,9 +236,6 @@ static int do_sign(struct strbuf *buffer)
 	if (finish_command(&gpg) || !len || len < 0)
 		return error("gpg failed to sign the tag");
 
-	if (len < 0)
-		return error("could not read the entire signature from gpg.");
-
 	return 0;
 }