Removal of “weak field” support (protolegacy feature)

[stapelberg]

Weak fields were the predecessor to extensions (many many years ago) and were entirely removed from Google’s production usage by now. (The corresponding field in descriptor.proto was always documented as “// For Google-internal migration only. Do not use.”)

Go Protobuf still contains support for weak fields behind the protolegacy build tag.

The protolegacy build tag was always documented as not being part of the compatibility agreement:

// WARNING: The compatibility agreement covers nothing provided by this flag.
// As such, functionality may suddenly be removed or changed at our discretion.
const ProtoLegacy = protoLegacy

We aim to remove support for weak fields from Go Protobuf. This issue is a heads-up for said removal.