Tests: Remove or fix Braintree donation feature

[danielsjoo]

DonationCheckoutServiceIntegrationTest fails with com.braintreegateway.exceptions.AuthenticationException because CircleCI does not set BT_MERCHANT_ID, BT_PUBLIC_KEY, and BT_PRIVATE_KEY. The test is marked as "currently broken" and asserts "Internal server error" (so it's deliberately looking for the 'wrong' answer)

Recommendation: Remove the donation feature and related tests if it’s unused.

Files: DonationCheckoutServiceIntegrationTest.java, BillingController.java, DonationCheckoutService.java, DonationGenerateUserTokenService.java