remove unused code. Make sure TaxJar Rate Calc overrides all other calcs. Hide order admin calculate tax button since it does not use TaxJar api.

git-svn-id: https://plugins.svn.wordpress.org/taxjar-simplified-taxes-for-woocommerce/trunk@972463 b8457f37-d9ea-0310-8a92-e5e31aec5664