Remove non-existing _DT from ordered dataclass

[guitvcer]

(Explain how this PR changes mypy.)

Fixes #18811

Remove _DT attribute that does not exist in runtime from ordered dataclass.

[sterliakov]

Won't this still keep creating _DT on @dataclass(eq=True)? The problematic bit is actually creating a fake TypeVarExpr and putting it in dataclass' symbol table.

Furthermore, this will actually break @dataclass(eq=False, order=True) definitions, because below self._api.fail() does not abort early, we'll still create methods referencing non-existent _DT, maybe leading to some crashes down the road.

[guitvcer]

Okay, I removed _DT from @dataclass(eq=True).

But I don't actually understand about @dataclass(eq=False, order=True) definitions, newbie in mypy:)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sterliakov]

Okay, this typevar was stored in class' symbol table since #6515. The question is how come that all our tests pass with this (rather huge) block removed, and there are zero primer hits. Note that attrs plugin and namedtuple semanal does the same, adding a typevar node to the modified class.

I feel like this will eventually fail in incremental mode, trying to consume existing __lt__ of some dataclass without corresponding TypeVar node. Using TypeVarType that doesn't have corresponding tvar node somewhere is just begging for trouble, but somehow I can't quickly create a reproducer that crashes readily. Can we really just get rid of it like this?..