vswitchd: Remove default controller config from Open_vSwitch table

vswitchd: Remove default controller config from Open_vSwitch table

An OpenFlow controller is normally associated with a bridge.  It was
possible to define a default controller in the Open_vSwitch table that
would be used if one was not associated with a bridge.  This was seldom
used and mostly just caused confusion.  This commit removes that
support, so an OpenFlow controller must always be associated with a
bridge.