ACPI: docs: Drop useless DSDT override documentation

Because https://01.org/linux-acpi

 web site has become permanently
inaccessible, the "Overriding DSDT" document in the kernel tree
pointing to it as the main source of information is useless (and
the config option name mentioned by it is incorrect), so drop it
and drop the pointer to it from the ACPI Kconfig.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>