Remove the data loading context when using PS servers in dophin-async

[wynot12]

The server evaluators of PS service do not require the data loading service. However, in the current implementation of dolphin-async, the driver submits a context with a data loading service, which do not load any data, to servers.

It was just for convenience of reusing code of dolphin-bsp, I guess. (Maybe the controller in dolphin-bsp also do not require data loading service.)

It's better to remove this unnecessary context from server-side to make a driver manage job easily and cleanly.