design_utils: Set port.net to null when disconnecting
design_utils: Set port.net to null when disconnecting

Without this the python bindings can't actually connect anything else
to a disconnected port since the assert in connect_ports will think
it's still connected

Signed-off-by: Sylvain Munaut <349f318521b45672c77207abf1a5ed60eb011db1@246tNt.com>

SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr