remove `JobDistributionProtocol` from `common_messages_sv2 `

[plebhash]

during the review process of #1035 we identified that common_messages_sv2 crate has some references to JobDistributionProtocol, which is just some legacy terminology that we forgot to remove after some spec changes

if we check stratumprotocol.org/specification we'll only find 3 subprotocols:

• Mining
• Job Declaration
• Template Distribution

but the Protocol enum declaration includes 4 different subprotocols: https://github.com/stratum-mining/stratum/blob/7841021e3f20280c62af119de5de58f421c6e428/protocols/v2/subprotocols/common-messages/src/setup_connection.rs#L293-L298

all references to JobDistributionProtocol should be cleaned up

[Shourya742]

@plebhash, can I take this on? This will help me understand the APIs in the protocol module, which might be useful during future refactoring.