Further clean up aggregation logic

[daniellepintz]

What does this PR do?

After I showed in #9145 (comment) that the aggregation logic in LightningLoggerBase is a no-op, due to the should_accumulate check in the fit loop, we can further clean up this code here without affecting anything.

Does your PR introduce any breaking changes? If yes, please list them.

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[daniellepintz]

Could someone rerun the TPU tests which failed?