SAS7BDAT parser: Drop unused instance variables
I'm suggesting this as an alternative to https://github.com/pandas-dev/pandas/pull/48004.

The `_page_count` attribute is read incorrectly (see https://github.com/pandas-dev/pandas/pull/48004) but it's also unused. I'm removing it here, along with some other unused things.

- [ ] closes #xxxx (Replace xxxx with the Github issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @jonashaag 