mean_and_variance: Drop always failing tests

mean_and_variance: Drop always failing tests

mean_and_variance_test_2 and mean_and_variance_test_4 always fail.
The input parameters to those tests are identical to the input parameters
to tests 1 and 3, yet the expected result for tests 2 and 4 is different
for the mean and stddev tests. That will always fail.

     Expected mean_and_variance_get_mean(mv) == mean[i], but
        mean_and_variance_get_mean(mv) == 22 (0x16)
        mean[i] == 10 (0xa)

Drop the bad tests.

Fixes: 65bc41090720 ("mean and variance: More tests")
Closes: https://lore.kernel.org/lkml/065b94eb-6a24-4248-b7d7-d3212efb4787@roeck-us.net/
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>