Remove legacy datetime datagen code, preserving still-relevant tests (#7383)

Progress on #5613


The only thing left was tests. There were only a few tests:

 - `test_basic_symbols`: preserved
- `unalias_symbols`: Names deduplication is handled by our data infra,
not by datetime-specific Option-ing
- `test_missing_append_items_support`:
#586 is still relevant, I
think, but this test is too deeply tied to the old data model. I'm not
sure it can still be written in the new one
- `test_basic_patterns`, `test_with_numbering_system`: I'm not 100%
sure, but as far as I can tell the new data model does not contain
skeleta in the same way.

So most seem no-longer-relevant to me.

A cleanup we can do after this is removing all "neo" naming from the
codebase.

Author: Manishearth