Fix remove unnecessary Promises from transaction write functions

[reedom]

Since the original Firestore's tx write functions are synchronous.

e.g.

set(documentRef: DocumentReference, data: DocumentData,
  options?: SetOptions): Transaction;

After this fix, I expect that some of my functions can be called within either tx or batch.
Prior to migrating to typesaurus, I had lines like:

async function incrementCounter(
  writer: Transaction | WriteBatch,
  kind: string,
  delta: number
)

I want to do the same but if the return type of each function differs, it's difficult to cope with.

[kossnocorp]

Great catch, thank you!