Avoiding setting filters during pmpro_checkout_level

[dparker1005]

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Changes proposed in this Pull Request:

Prevents conflicts that arise when pmpro_getLevelAtCheckout() is called for levels other than the level being purchased during a checkout page load. See #28 for more details.

Resolves #28

Relies on strangerstudios/paid-memberships-pro#3233

Once that core PR is merged, this PR should be updated to use the corresponding PMPro version in case that version is not PMPro v3.4.

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