Remove outdated elpy-pkg.el

[tarsius]

PR Summary

The information in <name>-pkg.el did not agree with the information in <name>.el. This pull-requests addresses that be removing the outdated <name>-pkg.el.

While the end-user package manager package.el expects a file <name>-pkg.el, this should only be generate by the package archive (such as GNU ELPA and MELPA), instead of being tracked in the upstream repository.

The tools used maintain the various *ELPA, do not use <name>-pkg.el as a data source, they only generate it it.

• elpa-admin.el, the tool used for GNU ELPA and NonGNU ELPA, does not use <name>-pkg.el as a data source and it never has.

• package-build.el, the tool used for Melpa, prefers <name>.el but currently falls back to get information missing from there from <name>-pkg.el instead. I am goint to change that; soon <name>-pkg.el will be ignored as a data source by this tool too.

PR checklist

• Commits respect our guidelines
• Tests are passing properly (see here on how to run Elpy's tests)

[tarsius]

You won the "first merge of this series" price. 🎉