When you no longer need a tax rate you have created you have the ability to archive that tax rate. By archiving the tax rate it will no longer show up as an option to select in the product or add-on create or edit screens. To archive a tax rate navigate to Settings > Tax Rates and Fees and find the tax rate you wish to archive in the active list. Select the Actions menu and select Archive.
It's important to note that any product or add-on that already has this tax rate applied to it will continue to use the tax rate until the rate has been removed from the product or add-on. Archived tax rates will not be available for selection in any new product or add-on created after the tax rate has been archived.