Remove wrong assertion in match checking.
This assertions is completely misguided, introduced by https://github.com/rust-lang/rust/pull/108504. The responsible PR is on beta, nominating for backport.
Instead of checking that this is not a `&&`, it would make sense to check that neither arms of that `&&` is a `let`. This seems like a lot of code for unclear benefit.
r? `@saethlin`