package/network-manager: don't require an external DHCP client

NetworkManager now has an internal DHCP client.  Therefor, there is no
need to select either the DHCPCD or DHCP_CLIENT package to get DHCP.

Remove the forced select of one of those packages.

The internal DHCP client has become NetworkManager's preferred DHCP
client, so it seems reasonable that it effectively becomes the default,
unless DHCPCD or DHCP_CLIENT are intentionally enabled.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>