WEB: Remove Extensibility section from Roadmap

[simonjayhawkins]

In #51471 (comment) @jbrockmendel wrote

No evidence the ecosystem wants this, closing.

in https://pandas.pydata.org/about/roadmap.html#roadmap-points-pending-a-pdep

pandas is in the process of moving roadmap points to PDEPs (implemented in August 2022). During the transition, some roadmap points will exist as PDEPs, while others will exist as sections below.

If we don't have actionable plans than can be written up as a PDEP, let's remove this from the roadmap.

---

In #27478 @TomAugspurger wrote

Do we want this? Roadmaps tend to go stale. How can we keep this up to date?

So maybe there are other sections that we also want to remove now?

@pandas-dev/pandas-core

[mroeschke]

I would generally support removing the entire roadmap.

There are some good ideas here, but I wouldn't say it's reflective of an active roadmap for the project.

[simonjayhawkins]

following suggestion by @mroeschke and thumbs up from @jbrockmendel i've removed all Roadmap points pending a PDEP.