Remove the redirection_date line from all existing site YAML

I paired with @rjc123 on this to do it in a script rather than me
spending quite a while removing them manually:

`while read line; do grep -v redirection_date $line >
site_list_redirection_date.txt; mv site_list_redirection_date.txt $line;
done < site_list.txt`

This is to be merged after https://github.com/alphagov/redirector/pull/918.