[clang] [NFC][analyzer] Do not spell out unreachable statement kinds in `shouldJustCallCheckers` (PR #223473)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

LGTM.

The removed code is all dead code, and `llvm_unreachable` calls in `ExprEngine::Visit` validate that it is dead.

This is also the more future-proof behavior for the unlikely case that some of these statement kinds get a "real" handler in `ExprEngine::Visit` (e.g. somebody implements modeling for some matrix operator): it that case that statement kind should also get `PreStmt` / `PostStmt` callbacks.

https://github.com/llvm/llvm-project/pull/223473