ACPICA: Tables: Remove old mechanism to validate if XSDT contains NULL entries.

With the NULL entry sanity check implemented, the XSDT validation is
useless because:
1. If XSDT contains NULL entries, it can be bypassed by the new sanity
   check mechanism;
2. If RSDP contains a bad XSDT address, invoking XSDT validation will still
   lead to a kernel crash.

This patch deletes the old XSDT validation solution and thus enables the
new NULL entry sanity check solution.

Note that if there are reports reporting regressions caused by the enabling
of the new feature and disabling of the old feature, this commit should be
bisected and reverted. Lv Zheng.

References: https://bugzilla.kernel.org/show_bug.cgi?id=73911
References: https://bugs.archlinux.org/task/39811


Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Reported-and-tested-by: Bruce Chiarelli <mano155@gmail.com>
Reported-and-tested-by: Spyros Stathopoulos <spystath@gmail.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>