ofproto: Remove support for OpenFlow-based management protocol.

ofproto: Remove support for OpenFlow-based management protocol.

Older versions of Open vSwitch supported a management protocol based on
OpenFlow message framing.  The current Open vSwitch instead uses the
OVSDB protocol for the same purposes.  We don't plan to support this older
protocol any longer, so this commit deletes support.

This commit also deletes the management_id column from the vswitch's
database schema.  The management_id was used by the older management
protocol to match up OpenFlow switch connections to management connections,
but the current implementation instead matches up connections based on
the datapath IDs exported by the configuration database.  In fact, the
OpenFlow connections had no way to actually export the management ID, so
this just deletes code that was essentially without function anyhow.