Remove Order#exclude_tax? as it is not used anywhere.

I'm working through VAT tax setup issues, and have noticed this method doesn't appear to be being used anywhere after all the tax work you've been doing @radar.  I think it is due to tax calculated on line item level instead of the order level now.

I didn't want to push right away in case you have any insight into it.  Once I have my head wrapped around all the tax included in price changes that happened in issues like https://github.com/spree/spree/pull/4327 I'll merge this if you don't first.

Also I believe https://github.com/spree/spree/pull/4327 can be closed as it's merged unless your leaving it open for people to provide feedback.