sd-future: drop redundant branch in test reader fiber

Both the error and the success path returned (int) n, so the check was
a no-op. Return the value directly.

CID#1660095

Follow-up for 7bc793e21f

src/libsystemd/sd-future/test-fiber-ops.c

@@ -422,8 +422,6 @@ static int loop_write_zero_blocking_reader_fiber(void *userdata) {
         _cleanup_free_ char *buf = malloc(ctx->total);
         ASSERT_NOT_NULL(buf);
         ssize_t n = loop_read(ctx->pipefd[0], buf, ctx->total, /* do_poll= */ true);
-        if (n < 0)
-                return (int) n;
         return (int) n;
 }