Tests: remove extraneous use of toColourspace

Author: lovell

test/unit/extractChannel.js

@@ -82,7 +82,6 @@ describe('Image channel extraction', function () {
     const output = fixtures.path('output.extract-alpha-2-channel.png');
     sharp(fixtures.inputPngWithGreyAlpha)
       .extractChannel('alpha')
-      .toColourspace('b-w')
       .toFile(output, function (err, info) {
         if (err) throw err;
         assert.strictEqual(1, info.channels);