diff: eliminate a useless lseek

fdopen with the "r" already position the stream at the beginning
of the file.

usr.bin/diff/diffreg.c

@@ -491,7 +491,6 @@ opentemp(const char *f)
 		}
 	}
 	close(ifd);
-	lseek(ofd, (off_t)0, SEEK_SET);
 	return (fdopen(ofd, "r"));
 }