tests: posix: common: remove sub-microsecond nanosleep test

Beyond the complexities of tick resolution on QEMU, this test is
invalid because POSIX nanosleep as currently implemented uses
k_busy_wait(ns / 1000) which means it's measuring the duration of
k_busy_wait(0) in ticks, which has no reasonable relation to 1 ns
regardless of tolerance.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>