fix(medusa): Cleanup Tax lines in case of a failed cart completion

[pKorsholm]

What

• remove tax lines in case an error is raised after they have been created

How

• fetch cart being completed along with items.tax_lines and shipping_methods.tax_lines
  • flapmap items and shipping_options to only the tax lines and remove them

Why

• hanging tax lines prevented any further action on the order

Fixes CORE-431

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[srindom]

Could we add a quick integration test that does:

• Complete cart
• Fail: e.g., because payment is not authorized
• Verify that tax lines are not present

[pKorsholm]

@srindom added tests for when the completion strategy throws and error and for when the payment status is pending (in both cases the hanging lines would cause this error)

[srindom]

LGTM! Great work 🎉

Good to merge once tests are fixed