Remove link to performance platform from the homepage
Remove link to performance platform from the homepage

Performance platform has been shut down, so this link now redirects to
https://data.gov.uk/dataset/731b25a8-0462-4a7d-aa3f-5a5d44ae26d2/historical-performance-platform,
which is not a particularly useful place to send users from the
homepage.

For now, probably best just to remove this.

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend