Remove deprecated APIs that never graduated to stdlib module

[ncoghlan]

• ContextStack was the original draft name for ExitStack
• ContextDecorator.refresh_cm was never made a public API in the stdlib version

[ncoghlan]

Keeping these around isn't a huge burden, since there's a diff from the stdlib version anyway to reinstate support for old Python versions. But there also isn't much point to keeping them around, since they've been deprecated for years now.