network: free DHCP client and friends in link_free()

network: free DHCP client and friends in link_free()

No functional change, at least now. Preparation for later commits.

But we are planning to extend KeepConfiguration= and also keep
addresses and so on assigned by other dynamic configuration protocol
like DHCPv6 or NDisc.
However, when link_free_engines() is called here, acquired addresses so
on by NDisc will be removed, even if link_stop_engines() handles
restarting networkd or KeepConfiguration= gracefully.
So, let's not free engines here, but free them later in link_free().
It is not necessary to be called here anyway.