Remove undefined variable in Stripe Gateway

[mircobabini]

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Changes proposed in this Pull Request:

Removed an undefined variable. Also useless, bc the called method can get the secretkey by itself.

How to test the changes in this Pull Request:

1. Open the code in PHPStorm
2. See the error showing up in the IDE

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?