Revert "board: x86: enable system timer clock frequency at run time"

This reverts commit 8eabf801.

https://github.com/zephyrproject-rtos/zephyr/pull/69705
Introduced a regression in main in which
tests/subsys/logging/log_timestamp
started failing. (See
https://github.com/zephyrproject-rtos/zephyr/issues/72344


for more info).
Let's revert the PR. It can be submitted after with the issue
fixed.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>