[Ada] Remove support for -gnatP and pragma Polling

[Ada] Remove support for -gnatP and pragma Polling

[Pierre-Marie de Rodat]

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This switch has never been very useful nor used in practice, so remove
the associated code complexity.

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

gcc/ada/

	* Makefile.rtl, gnat1drv.adb, expander.adb
	doc/gnat_rm/implementation_defined_pragmas.rst,
	doc/gnat_ugn/building_executable_programs_with_gnat.rst,
	doc/gnat_ugn/the_gnat_compilation_model.rst, exp_ch5.ads,
	exp_ch5.adb, exp_ch6.adb, exp_ch7.adb, exp_util.adb,
	exp_util.ads, libgnarl/s-osinte__aix.adb,
	libgnarl/s-osinte__android.adb, libgnarl/s-osinte__darwin.adb,
	libgnarl/s-osinte__gnu.adb, libgnarl/s-osinte__hpux-dce.adb,
	libgnarl/s-osinte__lynxos178.adb, libgnarl/s-osinte__posix.adb,
	libgnarl/s-osinte__qnx.adb, libgnarl/s-osinte__rtems.adb,
	libgnarl/s-osinte__solaris.adb, libgnarl/s-osinte__vxworks.adb,
	libgnarl/s-osinte__x32.adb, libgnarl/s-solita.adb,
	libgnarl/s-taasde.adb, libgnarl/s-taprob.adb,
	libgnarl/s-taprop__dummy.adb, libgnarl/s-taprop__hpux-dce.adb,
	libgnarl/s-taprop__linux.adb, libgnarl/s-taprop__mingw.adb,
	libgnarl/s-taprop__posix.adb, libgnarl/s-taprop__qnx.adb,
	libgnarl/s-taprop__solaris.adb, libgnarl/s-taprop__vxworks.adb,
	libgnarl/s-tarest.adb, libgnarl/s-tasini.adb,
	libgnarl/s-taskin.adb, libgnarl/s-taspri__dummy.ads,
	libgnarl/s-taspri__hpux-dce.ads, libgnarl/s-taspri__lynxos.ads,
	libgnarl/s-taspri__mingw.ads,
	libgnarl/s-taspri__posix-noaltstack.ads,
	libgnarl/s-taspri__posix.ads, libgnarl/s-taspri__solaris.ads,
	libgnarl/s-taspri__vxworks.ads, libgnarl/s-tassta.adb,
	libgnarl/s-tasuti.adb, libgnarl/s-tposen.adb,
	libgnat/a-except.adb, libgnat/a-except.ads,
	libgnat/s-dwalin.adb, libgnat/s-dwalin.ads,
	libgnat/s-mastop.ads, libgnat/s-soflin.adb,
	libgnat/s-stalib.adb, libgnat/s-stalib.ads,
	libgnat/s-stchop.adb, libgnat/s-stchop.ads,
	libgnat/s-stchop__limit.ads, libgnat/s-traceb.ads,
	libgnat/s-traent.adb, libgnat/s-traent.ads,
	libgnat/s-trasym.adb, libgnat/s-trasym.ads,
	libgnat/s-trasym__dwarf.adb, opt.adb, opt.ads, par-prag.adb,
	sem_prag.adb, snames.ads-tmpl, switch-c.adb, targparm.adb,
	targparm.ads, usage.adb: Remove support for -gnatP and pragma
	Polling.
	* gnat_ugn.texi: Regenerate.
	* libgnat/a-excpol.adb, libgnat/a-excpol__abort.adb: Removed.

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