Fix: use unique variable name when clearing old state data

[jdeepee]

What kind of change does this PR introduce? (check at least one)

• [x] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

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• [x] You consent that the copyright of your pull request source code belongs to Authlib's author.

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Fixes issue where state data for OAuth could be set on an incorrect session key due to overwriting the key value when removing old session state data.