Import Signer from ethers lib

In the past there was an issue with recognizing the `Signer` in the tbtc-v2.ts
lib. As a workaround I've used `Signer` from `@ethersproject/abstract-signer`.
The issue was fixed in `25d2de2ab89ee3c8428d7044f4f80fe53b15f750` so we can now
safely import SIgner from `ethers`.

typescript/src/lib/utils/ledger/ledger.ts

@@ -9,10 +9,8 @@ import { Hex } from "../../utils"
 import { AddressZero } from "@ethersproject/constants"
 import { Deferrable } from "@ethersproject/properties"
 import { getWalletAPIClient, getWindowMessageTransport } from "."
-import { Signer } from "@ethersproject/abstract-signer"
+import { Signer } from "ethers"
 
-// TODO: Investigate why it works with `Signer` from
-// `@ethersproject/abstract-signer` and not the one from `ethers` lib.
 export class LedgerLiveAppEthereumSigner extends Signer {
   private _walletApiClient: WalletAPIClient
   private _windowMessageTransport: WindowMessageTransport