Revert use of memory_args_by_pointer in i386 gdbarch_tdep.

Revert use of memory_args_by_pointer in i386 gdbarch_tdep.

This is no longer useful, as it was introduced to reuse the funcall
handling code in amd64-tdep.c in the context of x64-windows. But
we have since then changed the implementations to be completely
independent of each other.

This reverts the non-windows-specific part of the change called:
    amd64-windows: memory args passed by pointer during function calls.
(the x64-windows portion has already been reverted)

gdb/ChangeLog:

        Revert:
        * i386-tdep.h (gdbarch_tdep): Add field memory_args_by_pointer.
        * amd64-tdep.c (amd64_push_arguments): Add handling of architectures
        where tdep->memory_args_by_pointer is non-zero.