[Ada] Remove ASIS_Mode

[Pierre-Marie de Rodat]

In order to simplify the long term maintenance of the GNAT frontend,
support for generating trees for ASIS is removed from trunk. ASIS is
being phased out at this stage in favor of Libadalang.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.
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