Remove agent string

The agent string was inspired by the HTTP User-Agent header to serve as
a way to identify what's connecting to the cluster.  While there may
have been vision of other software connecting to a controller than the
Clusterio provided ones this seems highly unlikely to ever happen.

Remove redundant agent string for now.

docs/devs/protocol.md

@@ -107,7 +107,6 @@ Sent by the controller when the WebSocket connection has been opened.
 Host handshake for establishing a new connection session.
 
 - `token: string` - Authentication token.
-- `agent: string` - Human readable string of the software connecting.
 - `version: string` - The protocol version of the host, e.g. "2.0.0".
 - `name: string` - Name of the host.
 - `id: integer` - ID of the host.
@@ -119,7 +118,6 @@ Host handshake for establishing a new connection session.
 Control handshake for enstablishing a new connection session.
 
 - `token: string` - Authentication token.
-- `agent: string` - Human readable string of the software connecting.
 - `version: string` - The protocol version of the control, e.g. "2.0.0".
 
 #### `resume`
@@ -243,7 +241,6 @@ Host registering a new connection session.
     "type": "register_host",
     "data": {
         "token": "xyzabc123",
-        "agent": "Clusterio Host",
         "version": "2.0.0",
         "name": "Foo",
         "id": 123