"optional" is no longer a valid keyword in `proto3` files

[Qix-]

When trying to codegen with Prost, I came across an issue with chat.proto:

Explicit 'optional' labels are disallowed in the Proto3 syntax. To define 'optional' fields in Proto3, simply remove the 'optional' label, as fields are 'optional' by default.

I'm not familiar with protobuf's newest syntax/formats but it appears that there's some legacy syntax in here that isn't otherwise being picked up by the Python codegen tools that is being caught by other tools in the ecosystem.

Removing them all made Prost happy.