Remove extraneous $wpdb globalization

There were a handful of functions containing legacy $wpdb globals.  These can be safely removed.
Google Analytics, Add to cart event uniformity for shop and single.
add action woocommerce_variable_product_sync to variable sync
Merge pull request #3537 from helgatheviking/new-action/woocommerce_variable_product_sync

New action/woocommerce variable product sync
Merge pull request #3533 from kloon/master

Google Analytics, Add to cart event uniformity for shop and single.
Merge pull request #3532 from JustinSainton/silly-wpdb

Remove extraneous $wpdb globalization