[Ada] Remove Ada.Strings.Text_Output and child units

[Pierre-Marie de Rodat]

The GNAT-defined package Ada.Strings.Text_Output has been replaced by
the Ada-defined package Ada.Strings.Text_Buffers.

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

gcc/ada/

        * libgnat/a-stobbu.adb, libgnat/a-stobbu.ads,
        libgnat/a-stobfi.adb, libgnat/a-stobfi.ads,
        libgnat/a-stoubu.adb, libgnat/a-stoubu.ads,
        libgnat/a-stoufi.adb, libgnat/a-stoufi.ads,
        libgnat/a-stoufo.adb, libgnat/a-stoufo.ads,
        libgnat/a-stouut.adb, libgnat/a-stouut.ads,
        libgnat/a-stteou.ads: Delete files.
        * Makefile.rtl, impunit.adb: Remove references to deleted files.

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