Update Landcover raster layer to use publicly available mosaic

The LC.tif for each country is mosiac-ed and transformed into a COG [here](https://github.com/developmentseed/covid-wb-api/blob/master/etl/load_tifs.sh#L41-L46). A colormap is also applied to the Tif. The byte values in the Tifs do not exactly match up with the legend for the Globcover product, and that's noted in a couple places in TODOs.

I suggested, and Benjamin Stewart from WB gave approval via email, to instead use the premade Globcover Tif that is available here: http://due.esrin.esa.int/page_globcover.php

- [ ] yank the code in the etl scripts
- [ ] remove TODO in code about fixing the legend or byte values for the product
- [ ] convert the Globcover mosaic to COG
- [ ] upload to s3 and confirm it works with our raster tiling endpoints