Tests: remove extraneous use of toColourspace

docs/src/content/docs/api-channel.md

@@ -61,6 +61,8 @@ const rgba = await sharp(rgb)
 
 Extract a single channel from a multi-channel image.
 
+The output colourspace will be either `b-w` (8-bit) or `grey16` (16-bit).
+
 
 **Throws**:
 

lib/channel.js

@@ -74,6 +74,8 @@ function ensureAlpha (alpha) {
 /**
  * Extract a single channel from a multi-channel image.
  *
+ * The output colourspace will be either `b-w` (8-bit) or `grey16` (16-bit).
+ *
  * @example
  * // green.jpg is a greyscale image containing the green channel of the input
  * await sharp(input)

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);