fix!: delete visibleContent check to remove get-pixels dependency

When migration Financial-Times/next-preflight#723 to Tool Kit, the snyk
check failed as the `get-pixels` dependency of `n-test` has a couple of
security advisories open (see link below). Unfortunately the get-pixels
dependency has not been updated in a couple of years and show no sign
of resolving this issue.

https://app.snyk.io/org/customer-products/project/4d2f1c5e-1868-477b-afcf-70879ce6b2d9/pr-check/7ee89281-dbc5-4879-801e-80432000340d

This dependency is only for the `visibleContent` check which isn't
configured in any repository in the Financial-Times org:

https://github.com/search?q=org%3AFinancial-Times%20visibleContent&type=code

Therefore we can remove this functionality. I spoke with Arjun to double
check and he informed me that this was an experimental feature that
never took off which can be removed.

README.md

@@ -107,10 +107,6 @@ urls: {
 		content: (content) => {
 			return content.includes('some-text');
 		},
-		visibleContent: {
-		  contentSelector: '.headline, .image, .standfirst'
-		  threshold: 30 // % of viewport that should be visible content
-		},
 		performance: true, //checks firstPaint/firstContentfulPaint against baseline. default = 2000, or can specify.
 		description: 'Each test may have an optional description. It will display when the test result is reported',
 	}

lib/checks/index.js

@@ -8,6 +8,5 @@ module.exports = {
 	pageErrors: require('./page-errors'),
 	performance: require('./performance'),
 	responseHeaders: require('./response-headers'),
-	screenshot: require('./screenshot'),
-	visibleContent: require('./visible-content')
+	screenshot: require('./screenshot')
 };