Plane: move `have_autoenabled_fences` to be private in `ModeTakeoff` #15450

Job	Run time
build (check_autotest_options)	1m 10s
build (param_parse)	46s
build (python-cleanliness)	40s
build (astyle-cleanliness)	47s
build (validate_board_list)	35s
	3m 58s

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