[internal] Remove packet->serialize()

Remove QRemoteObjectPacket::serialize() functions in
favor of direct serialize<Type>Packet() functions.

This remove the need to copy data into the Packet type
before serializing.

Change-Id: Ic5953a6e63731b64e79ffa5bef38a5b62179dd35
Reviewed-by: Björn Breitmeyer <bjoern.breitmeyer@kdab.com>