These units are not required anymore as oldest compiler version allowed
for bootstrap does not need them.

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

gcc/ada/

	* libgnat/s-sopco3.ads, libgnat/s-sopco3.adb: Remove.
	* libgnat/s-sopco4.ads, libgnat/s-sopco4.adb: Remove.
	* libgnat/s-sopco5.ads, libgnat/s-sopco5.adb: Remove.
	* libgnat/s-strops.ads, libgnat/s-strops.adb: Remove.
	* Makefile.rtl (ADA_EXCLUDE_SRCS): Remove occurences of removed
	units.
	* gcc-interface/Make-lang.in (ada/sdefault.o): Remove
	dependencies on removed units.
	(GNATBIND_OBJS): Remove occurences of removed units.