Remove optional label in witnet.proto field

[drcpu-github]

There is an optional field in witnet.proto which should not be necessary as all fields are optional by default. Rustc 1.77 even refuses to build this:

witnet.proto:304:14: 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.
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