Remove class-object assertion from defaults test

[JelleZijlstra]

#2204 rewrote many defaults tests because type checkers are not required to infer a class expression as having a type[C] type, but the assertion for slice in generics_defaults_referential.py was missed. The same rationale applies here. I simply removed the line because the next line with slice() adequately tests the defaults.

[AlexWaygood]

I believe the reason why I didn't change this one in #2204 was that we emit assert-type-unspellable-subtype for this one in ty instead of type-assertion-failure (since both the asserted type and the inferred type are fully static types), and we just switch off assert-type-unspellable-subtype when running ty on the conformance suite

[JelleZijlstra]

we just switch off assert-type-unspellable-subtype when running ty on the conformance suite

I feel like we shouldn't be doing that, it weakens the assert_type tests. But I guess we can gradually burn down the cases where we need it.