Use reject, rather than delete_if, for products_available_on_downloads_site
Use reject, rather than delete_if, for products_available_on_downloads_site

`delete_if` will modify the in-memory product_map, meaning that
if you call `PRODUCT_MATRIX.products_available_on_downloads_site`,
`PRODUCT_MATRIX` will only include products that are on the download
site.

`#reject` will simply return a new array, which is technically what I
think we want.

Signed-off-by: Tom Duffield <96835dd8bfa718bd6447ccc87af89ae1675daeca@chef.io>

chef/mixlib-install,chef/mixlib-install,chef/mixlib-install