Remove unreliable parts of rt/tst-cpuclock2

This is a follow-up to the tst-cpuclock1.c change here:
9a29f1a2

This test, like tst-cpuclock1, may fail on heavily loaded VM
servers (and has occasionally failed on the 32bit trybot),
so tests that rely on "wall time" have been removed.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>