[Ada] Remove ASIS_Mode

[Ada] Remove ASIS_Mode

2020-06-02  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* atree.ads, checks.adb, contracts.adb, debug.adb, einfo.ads,
	exp_ch3.adb, exp_util.adb, expander.ads, expander.adb,
	frontend.adb, gnat1drv.adb, itypes.adb, lib.ads, namet.ads,
	opt.adb, opt.ads, par-prag.adb, repinfo.ads, sem_aggr.adb,
	sem_aux.ads, sem_case.ads, sem_ch10.adb, sem_ch12.adb,
	sem_ch13.adb, sem_ch3.adb, sem_ch4.adb, sem_ch6.adb,
	sem_dim.adb, sem_elab.adb, sem_prag.adb, sem_prag.ads,
	sem_res.adb, sem_type.adb, sem_util.adb, sinfo.ads, stand.ads,
	tree_io.ads: Remove references to ASIS_Mode.