Delete pages related to uk-met-covid-19 since service no longer available

[tam203]

Theis pages should be taken down:

• https://microsoft.github.io/AIforEarthDataSets/data/uk-met-covid-19.html

And associated repo items deleted:

• data/uk-met-covid-19.ipynb
• data/uk-met-covid-19.md

As per the blog post referenced in the doc: https://medium.com/informatics-lab/met-office-and-partners-offer-data-and-compute-platform-for-covid-19-researchers-83848ac55f5f this service has been retired and the data is no longer available.