Revert "Deprecated casting an object-dtype index of datetime objects to DatetimeIndex in the Series constructor (23598)"
This **partially** reverts commit 90a61350b1be7a200238212bd8d118a25a4d3d95.

- [ ] closes #38182
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

fine by me. cc @jbrockmendel  If the idea is to un-revert following 1.2, sure. > If the idea is to un-revert following 1.2, sure.

from https://github.com/pandas-dev/pandas/issues/38182#issue-753330794

> Users should not see unrelated deprecation warnings from the internals of pandas that they can't do anything about themselves, so IMO we should either fix those for 1.2, or delay the deprecationg for 1.3

not fixed, so delaying to 1.3 ok great, thanks @simonjayhawkins 

yeah we can unrevert this for 1.3, but i think ok to release once this hits. @meeseeksdev backport 1.2.x