eliminate unused styles .bg-1 .bg-2 .bg-3 .bg-4 .bg-5

We found some unused style classes.

This is how I searched, as well as in rubymine:
`find . -type f | grep -v cache/assets | grep -v ./log  | xargs  grep .bg- --color`

The results include some things like bg-color but I didn't want to complicate the grepping.
https://gist.github.com/apelade/bd5b4393b2b078914221