[llvm] [CloneFunction] Remove check that is no longer necessary (PR #92577)

[Danila Malyutin via llvm-commits]

danilaml wrote:

@nikic Yeah, wasn't able to come up with the test. Another thing is that upstream simplifyCall currently only handles poison/undef callee (wasn't skipped due to `CB->getCalledFunction()` check), intrinsics (wasn't skipped by the check) and also attempt to constant fold call (was already done before), but I still think it's good to remove this "dead" code with outdated comment.

https://github.com/llvm/llvm-project/pull/92577