DISC: Remove DTA/TDA/PA `freq`
For PA, freq is redundant with dtype.  For DTA/TDA, it means something very different from what it means for PA.  The only thing it is really used for is _time_shift (and along with it, add/sub of integers, which is deprecated in DTI/TDI anyway)

Without `freq` going into the constructors, DTA/TDA/PA would all just need `dtype`, so some simplification/unification could be done.

frequency validation in `__init__` could be saved for DTI/TDI and omitted from DTA/TDA.

If we ever come to our senses and allow 2D EAs (so we can simplify a ton of internals), freq stops making sense for 2D, so would be unused in those cases anyway.