ACPICA: Interpreter: Remove temporary code for External() opcode

ACPICA commit f2d349f8a11efc0f438ad6903564f3a6755dc6b9

The interpreter should never see this opcode (it is used by
disassemblers), so the final implementation is to return an
error.

Link: https://github.com/acpica/acpica/commit/f2d349f8


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