remove subdir_do recursive make command in gdbserver/Makefile.in

remove subdir_do recursive make command in gdbserver/Makefile.in

This fixes:

    % [...]/gdbserver/configure
    % make distclean
    [...]
    rm -f amd64-avx.c amd64-avx-linux.c
    rm -f i386-mmx.c i386-mmx-linux.c
    make[1]: Entering directory `/[...]'
    make[1]: *** No rule to make target `subdir_do'.  Stop.
    make[1]: Leaving directory `[...]'
    make: *** [distclean] Error 2

gdb/gdbserver/ChangeLog:

        * Makefile.in (maintainer-clean realclean distclean): Remove
        "make ...  subdir_do" command.