xtensa: Use epilogue_completed rather than cfun->machine->epilogue_done

In commit ad89d820, an "epilogue_done" member was added to the
machine_function structure, but it is sufficient to use the existing
"epilogue_completed" global variable.

gcc/ChangeLog:

	* config/xtensa/xtensa-protos.h
	(xtensa_use_return_instruction_p): Remove.
	* config/xtensa/xtensa.cc
	(machine_function): Remove "epilogue_done" field.
	(xtensa_expand_epilogue): Remove "cfun->machine->epilogue_done" usage.
	(xtensa_use_return_instruction_p): Remove.
	* config/xtensa/xtensa.md ("return"):
	Replace calling "xtensa_use_return_instruction_p()" with inline code.