ACPICA: Remove unused ACPI_FREE_BUFFER macro. No functional change.

This macro is no longer used by ACPICA and it is not public.
Also update comments related to the use of ACPI_ALLOCATE_BUFFER and
the use of acpi_os_free (kfree is equivalent and prefered in the
kernel) to free the buffer.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>