Remove Imperial ensemble forecast files from data-raw/ folder

[jarad]

All team forecasts should be in a subdirectory of data-raw/, but these files

https://github.com/reichlab/covid19-forecast-hub/blob/master/data-raw/2020-04-19-Imperial-ensemble1.csv
https://github.com/reichlab/covid19-forecast-hub/blob/master/data-raw/2020-04-19-Imperial-ensemble2.csv

are directly in the data-raw/ folder.

I would create a pull request, but these files have differences to the files in the data-raw/Imperial subdirectory, so I'm not sure which versions should be preserved.