Don't special-case ColumnDataSource in glyph.coffee

I'm not sure why this code was there, but `data2` isn't set anywhere and after recent changes to CDX, plots don't work anymore without this "fix". I'm looking forward to feedback from anyone who understands this part of bokeh. If there is a reason for this code (with appropriate fixes), then I think that `==` should be replaced with appropriate implementation of `<:` (subtype) type relation.