Checkbox Still Available for Discounted Level

[ideadude]

After entering a discount code that makes the level free, the auto renewal checkbox is still visible.

This happened with a sponsored-member discount code.

[dparker1005]

Wrote up this Gist quickly that should remove the checkbox for all levels when a discount code is entered, but needs testing to ensure it acts correctly with paid levels (meaning it completely deactivates the auto-renewal checkbox functionality for the payment plan designated by the discount code). If giving out discount codes for free memberships, though, this should work:
https://gist.github.com/dparker1005/b7bd4ff05e606d2ef7ee8d04ada430d1

After testing, this should be a quick PR into the main plugin.