Remove ParquetSource::pruning_predicate

[alamb]

Is your feature request related to a problem or challenge?

• As described on Add dynamic pruning filters from TopK state #15301 in https://github.com/apache/datafusion/pull/15301/files#r2021619638

I suggest we change this code to always return None (and remove the page pruning predicate from the source).

Otherwise I predict that we'll end up with this field slowly bitrotting (aka stop working but not covered by tests)

ParquetSource::pruning_predicate is no longer used after #15301

Describe the solution you'd like

so we should remove the actual field (while leaving the deprecated methods)

Describe alternatives you've considered

No response

Additional context

No response