Revert "implement TimedeltaArray asm8, to_timedelta64 (#23205)"
This reverts commit 6e507133916fc5a8434e21eb267043fc9a3ba8d7 (https://github.com/pandas-dev/pandas/pull/23205)
Hello @jorisvandenbossche! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimes.py`](https://github.com/jorisvandenbossche/pandas/blob/a5020955ab9fbcde9e37b93063d1922ac0143827/pandas/core/arrays/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/arrays/timedeltas.py`](https://github.com/jorisvandenbossche/pandas/blob/a5020955ab9fbcde9e37b93063d1922ac0143827/pandas/core/arrays/timedeltas.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arrays/test_datetimelike.py`](https://github.com/jorisvandenbossche/pandas/blob/a5020955ab9fbcde9e37b93063d1922ac0143827/pandas/tests/arrays/test_datetimelike.py) !

