TimedeltaArray freq validation without _from_sequence
This aligns `TimedeltaArray.__init__` with `DatetimeArray.__init__`, using the same approach how @jbrockmendel now handled the freq validation in `DatetimeArray` in the merged https://github.com/pandas-dev/pandas/pull/24686 
First commits removes usage of `from_sequence` (from https://github.com/pandas-dev/pandas/pull/24666), second commit adds the freq validation as in https://github.com/pandas-dev/pandas/pull/24686 

So related to the discussion at the end https://github.com/pandas-dev/pandas/pull/24666 (and a partial revert of that). This makes the `TimedeltaArray` constructor again more restricted (only accepts correctly typed (int or timedelta64) containers. 
Until we decide on the constructors in general (https://github.com/pandas-dev/pandas/issues/24684), I would prefer to keep them strict: it is always easier to later expand functionality (and then also test it), than remove functionality.