removing ec from post_updates list when deleted.

removing ec from post_updates list when deleted.

    When ec's ref_count is 0, _e_client_free() is called. However,
    e_comp->post_updates has the reference of ec. So it doens't make
    sense that _e_client_free calls e_comp_post_update_purge. To make
    ec's ref_count 0, we should call e_comp_post_update_purge first.
    Furthermore, we call e_comp_object_render_update_del(ec->frame)
    in _e_client_del. So _e_client_del seems the proper position to
    call e_comp_post_update_purge

Change-Id: I906101300dec08f7bf18f18428443efa47f7afec