configure: Remove unused CONFIG_ZERO_MALLOC setting

configure: Remove unused CONFIG_ZERO_MALLOC setting

CONFIG_ZERO_MALLOC was only used in qemu-malloc.c and
this file has been removed with the following commit:

	41a748265f4879b52b0e87ff9c93bed975163886
	Remove qemu_malloc/qemu_free

So we don't need this configuration setting anymore.
This patch also removes the z_version variable, since
this is now also not needed anymore.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <1465398683-3152-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>