Update NEWS.rst for 1.6.0
Bump version: 1.5.0 → 1.6.0
Bump version: 1.6.0 → 1.6.50
Simplify elpy--sort-and-strip-duplicates

There's a `delete-dups` function in Emacs, and it exists since 24.1.
Who knew.
Remove idomenu.

The default imenu definition is quite fine, and idomenu can be installed
trivially without much effort if the user wants it.