Revert "add case-specific stopwords"

This reverts commit 6c804bba6cc7c6a10cd0dacb3b1bb255527b1075.

I'm not sure what happened on my box exactly, but Pod::Spell is pretty clear
that stopwords in all lower-case are applied case-insensitively, so there
should not be any need for these duplicated entries.
avoid rebuilding the dist between running t/ tests and xt/ tests