Remove last unreachable block from mypyc code

[sterliakov]

Supplements #19050. The old version did certainly contain unreachable code, as otherwise that branch would have crashed, there is no type_args attribute. This block was introduced in #17027 - probably as a future-proof expansion to generic primitive operations, but it makes no sense now, this logic can be added back should it become necessary. cc @JukkaL as the original author

[A5rocks]

I actually ran into this when making mypy check unreachable code and replaced the unreachable branch with an assert False so I'm very sure this is right!