Investigate removing subpath fallback files

[smaye81]

Currently, we have fallback files in packages/connect for consumers that don't support subpath exports (protocol.js, protocol-connect.js, protocol-grpc.js, and protocol-grpc-web.js).

However, these files may not be needed as it is basically supporting Node.js v10 module resolution. Since Node 10 has been EOL for more than two years, we could potentially remove these altogether.

This could potentially break some bundlers, so we'd have to conduct an investigation into whether we could safely remove them.

For further context, see #838 (comment)

[timostamm]

We have removed the fallback files in bufbuild/protobuf-es#804.
If this does not cause any issues, we can safely do it here as well.