Refactor eth_sendTransaction handler (#5619)

The handler for `eth_sendTransaction` was previously spread between
`RPCMethodMiddleware.ts` and the static middleware of
`web3-provider-engine`. Instead it is all handled in
`RPCMethodMiddleware.ts` now.

This should have no functional impact. It is difficult to trace through
`web3-provider-engine`, but this case is one of the easier ones because
the static middleware is run first, and in this case it will always end
the request.

This relates to #5513

app/core/Engine.ts

@@ -26,7 +26,6 @@ import { PreferencesController } from '@metamask/preferences-controller';
 import {
   Transaction,
   TransactionController,
-  WalletDevice,
 } from '@metamask/transaction-controller';
 import { GasFeeController } from '@metamask/gas-fee-controller';
 import { ApprovalController } from '@metamask/approval-controller';
@@ -113,27 +112,6 @@ class Engine {
 
       const networkController = new NetworkController(networkControllerOpts);
       networkController.providerConfig = {
-        static: {
-          eth_sendTransaction: async (
-            payload: { params: any[]; origin: any },
-            _next: any,
-            end: (arg0: undefined, arg1: undefined) => void,
-          ) => {
-            const { TransactionController } = this.context;
-            try {
-              const hash = await (
-                await TransactionController.addTransaction(
-                  payload.params[0],
-                  payload.origin,
-                  WalletDevice.MM_MOBILE,
-                )
-              ).result;
-              end(undefined, hash);
-            } catch (error) {
-              end(error);
-            }
-          },
-        },
         getAccounts: (
           end: (arg0: null, arg1: any[]) => void,
           payload: { hostname: string | number },