Revert "sphinx: ada: port to Sphinx"

gcc/ada/doc/Makefile

gcc/ada/doc/gnat-style.rst

gcc/ada/doc/gnat-style/conf.py

gcc/ada/doc/gnat-style/gnu_free_documentation_license.rst

gcc/ada/doc/gnat_rm.rst

gcc/ada/doc/gnat_rm/conf.py

gcc/ada/doc/gnat_rm/gnu_free_documentation_license.rst

gcc/ada/doc/gnat_rm/security_hardening_features.rst

gcc/ada/doc/gnat_ugn.rst

gcc/ada/doc/gnat_ugn/conf.py

gcc/ada/doc/gnat_ugn/gnu_free_documentation_license.rst

gcc/ada/doc/gnat_ugn/platform_specific_information.rst

gcc/ada/doc/share/conf.py

gcc/ada/doc/share/gnu_free_documentation_license.rst

gcc/ada/doc/share/latex_elements.py

gcc/ada/gcc-interface/Make-lang.in

gcc/ada/gnat-style.texi

gcc/ada/gnat_rm.texi

gcc/ada/gnat_ugn.texi