REF: prelim for fixing array_to_datetime
`tslib.array_to_datetime` has an unused `format` kwarg.  This gets rid of it.

`to_datetime` (or more specifically, `_convert_listlike_datetimes`) has a bunch of boxing logic inside a try/except that doesn't need to be, obscures what Exception we're trying to catch.  This moves that code outside the try/except and de-indents it.  This overlaps with #23675.

xref #23722 (the conversation there less than the OP)  cc @mroeschke 