fix: Remove incorrect,unreachable and unused code (#1983)

* Prevent Serialization of firestore

The toJSON function spelling name is incorrect therefore this method is not triggered when using JSON.stringify on firestore objects

* Remove unused code

This code is never invoked, the spelling of toJSON is also wrong (  it is toJson here) 
Also the Firestore Class toJSON method works well now for protecting the private keys of firestore instance

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Co-authored-by: Mila <107142260+milaGGL@users.noreply.github.com>

dev/src/index.ts

@@ -745,13 +745,6 @@ export class Firestore implements firestore.Firestore {
     }
 
     this._settings = settings;
-    this._settings.toJson = function () {
-      const temp = Object.assign({}, this);
-      if (temp.credentials) {
-        temp.credentials = {private_key: '***', client_email: '***'};
-      }
-      return temp;
-    };
     this._serializer = new Serializer(this);
   }