BLD: since we already use setuptools, let's remove the optional logic in setup.py

[jreback]

no need to have the branching in setup.py we always have setuptools available.

Besides cleaning up setup.py does something else should be done? Are there any caveats regarding pandas build process?

[jreback]

no i think this all that is needed; we do sufficient testing on building in the CI

@jreback Do you think we could have Build subsection in whatsnew docs for BLD related entries? Or do they fall under one of existing subsections?

[jbrockmendel]

the try/except of import setuptools also includes import pkg_resources. Is it also always the case that we always have pkg_resources?