Continue de-nesting core.ops
- Move `isinstance(other, ABCDataFrame)` checks to consistently be the first thing checked in Series ops

- Remove `force` kwarg, define it in the one place it is used.

- Remove kludge for `PeriodIndex`

- Handle categorical_dtype earlier in arith_method_SERIES, decreasing complexity of the closure.

- Handle scalar na other earlier in _comp_method_SERIES, decreasing complexity of the closure.

- Remove broken broadcasting case from _arith_method_FRAME (closes #19421)