[clang] [clang] NFC, simplify the attr-lifetime-capture-by.test (PR #118001)

[Haojian Wu via cfe-commits]

hokein wrote:

> Can you not remove the check-not. It makes sure we are matching all the capture by attr.

I’m not sure they're necessary here. We’ve already verified that the relevant parameters have the attached capture_by attribute using CHECK-NEXT, I think it should be sufficient. The additional `CHECK-NOT` lines feel a bit noisy to me and can be confusing when updating this test.

https://github.com/llvm/llvm-project/pull/118001