random: remove rand32_timestamp.c

This is a copy of rand32_timer.c that uses
z_do_read_cpu_timestamp32() instead of k_cycle_get_32(),
with some logic to ensure different values when called in
rapid succession missing.

Like the other driver, its reported values are not random,
it's a testing generator only.

This appears to have no advantages over rand32_timer.c,
just remove it. In QEMU emulation, the reported TSC values
tend to have the lowest five bits zeroed.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>