chore(python): remove dataframe consortium standard api entrypoint

[MarcoGorelli]

As discussed on Discord

In short:

• I've left the project
• the "standard api" wouldn't allow for efficient polars code (due to unwilligness to separate eager vs lazy apis and unwillingness to consider expressions)
• plenty of significant projects are taking steps, or working towards, native Polars support (proving that Polars doesn't need a standard api in order to gain adoption)

Sorry for the (3?) CI issues it caused. On the plus side, it did help find some minor bugs in Polars (all promptly addressed!)

Calling it off then - it can always be reconsidered in the future in the unlikely scenario that it comes useful