asset-s: Remove erroneous addition of API key

It was added due to the introduction of the use of the TakerRequest type
from @0x/quote-server, as it's included in that structure.  However,
that structure is primarily used within the quote server as the output
of parameter parsing, and that parsing routine transforms the API key
HEADER from the request into the TakerRequest type.  In short, the API
key is input as a header, not a parameter.

Addresses review comment
https://github.com/0xProject/0x-monorepo/pull/2582#discussion_r437624647

packages/asset-swapper/test/quote_requestor_test.ts

@@ -37,7 +37,6 @@ describe('QuoteRequestor', async () => {
             // tslint:disable-next-line:array-type
             const mockedRequests: MockedRfqtFirmQuoteResponse[] = [];
             const expectedParams: TakerRequest = {
-                apiKey,
                 sellTokenAddress: takerToken,
                 buyTokenAddress: makerToken,
                 sellAmountBaseUnits: new BigNumber('10000'),
@@ -202,7 +201,6 @@ describe('QuoteRequestor', async () => {
             // tslint:disable-next-line:array-type
             const mockedRequests: MockedRfqtIndicativeQuoteResponse[] = [];
             const expectedParams: TakerRequest = {
-                apiKey,
                 sellTokenAddress: takerToken,
                 buyTokenAddress: makerToken,
                 sellAmountBaseUnits: new BigNumber('10000'),
@@ -297,7 +295,6 @@ describe('QuoteRequestor', async () => {
             // tslint:disable-next-line:array-type
             const mockedRequests: MockedRfqtIndicativeQuoteResponse[] = [];
             const expectedParams: TakerRequest = {
-                apiKey,
                 sellTokenAddress: takerToken,
                 buyTokenAddress: makerToken,
                 buyAmountBaseUnits: new BigNumber('10000'),