[Ada] Remove ASIS tree generation

[Ada] Remove ASIS tree generation

2020-06-03  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* aspects.adb, aspects.ads, atree.adb, atree.ads, elists.adb,
	elists.ads, fname.adb, fname.ads, gnat1drv.adb, lib.adb,
	lib.ads, namet.adb, namet.ads, nlists.adb, nlists.ads, opt.adb,
	opt.ads, osint-c.adb, osint-c.ads, repinfo.adb, repinfo.ads,
	sem_aux.adb, sem_aux.ads, sinput.adb, sinput.ads, stand.ads,
	stringt.adb, stringt.ads, switch-c.adb, table.adb, table.ads,
	uintp.adb, uintp.ads, urealp.adb, urealp.ads (Tree_Read,
	Tree_Write): Remove generation of ASIS trees.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Remove -gnatt and -gnatct documentation.
	* gnat_ugn.texi: Regenerate.
	* tree_in.ads, tree_in.adb, tree_io.ads, tree_io.adb,
	tree_gen.ads, tree_gen.adb, stand.adb: Remove.
	* gcc-interface/Makefile.in, gcc-interface/Make-lang.in: Remove
	references to tree_gen.o tree_in.o tree_io.o.