chore(hash): delete capnp source (#9118)

### Description

We're not making use of the Go `capnp` bindings anymore so we can remove
the source and the fixups from our repo.

### Testing Instructions

Hashes stay stable in the integration tests

Author: chris-olszewski

crates/turborepo-lib/build.rs

@@ -8,7 +8,6 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         );
     let capnpc_result = capnpc::CompilerCommand::new()
         .file("./src/hash/proto.capnp")
-        .import_path("./src/hash/std") // we need to include the 'stdlib' for capnp-go
         .default_parent_module(vec!["hash".to_string()])
         .run();
 

crates/turborepo-lib/src/hash/proto.capnp

@@ -1,10 +1,5 @@
-using Go = import "/go.capnp";
-
 @0xe1dde60149aeb063;
 
-$Go.package("capnp");
-$Go.import("capnp");
-
 struct TaskHashable {
     globalHash @0 :Text;
     taskDependencyHashes @1 :List(Text);