Remove arg that is only ever used as NPY_UNSAFE_CASTING
Several functions from src/datetime are only ever called with the casting rule NPY_UNSAFE_CASTING.  By getting rid of that dummy argument, the remaining code gets simplified quite a bit.

This PR removes that argument, then removes code that this renders unreachable or unused.  It also removes several commented-out functions.

There are a couple of other never-used args; taking these one at a time.
