Remove "add-shared-symbol-files", "dll-symbols" and "assf" commands doc.

Remove "add-shared-symbol-files", "dll-symbols" and "assf" commands doc.

This patch removes documentation from some commands whose support has
been recently removed.

gdb/ChangeLog:

        * NEWS: Document removal of "dll-symbols", "add-shared-symbol-files"
        and "assf" commands.

gdb/doc/ChangeLog:

        * gdb.texinfo (Files): Remove documentation of the
        "add-shared-symbol-files" and "assf" commands.
        (Cygwin Native): Remove documentation of the "dll-symbols"
        command.