fix(wpcarro/ynab): Remove .skip-subtree

**TL;DR:**
- Delete half-baked packaging attempts (`job.nix`, `token.nix`).
- Ensure golang code compiles.
  - Some "packages" were being treated like "programs" presumably for
    debugging/testing purposes back when I was working on this. Make those
    behave like libraries.
  - Remove stale imports.
  - Fix syntax errors.
  - Miscellaneous other chores.
- Drop `shell.nix` and `use_nix` directive.

Change-Id: I63c275680bac55a3cad3b9cb48d51cdc431fbe48
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7318
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

users/wpcarro/tools/monzo_ynab/ynab/serde.go

@@ -1,10 +1,9 @@
 // This package hosts the serialization and deserialization logic for all of the
 // data types with which our application interacts from the YNAB API.
-package main
+package ynabSerde
 
 import (
 	"encoding/json"
-	"fmt"
 	"time"
 )
 
@@ -43,10 +42,3 @@ func deserializeTx(x string) (*Transaction, error) {
 	err := json.Unmarshal([]byte(x), target)
 	return target, err
 }
-
-func main() {
-	target, _ := deserializeTx(tx)
-	out, _ := serializeTx(target)
-	fmt.Println(out)
-	fmt.Println(ynabOut)
-}