[clang] [clang][ASTImporter] Remove trailing return testing on lambda proto (PR #101031)

[Balázs Kéri via cfe-commits]

balazske wrote:

> Here can `ToProcess.append(CurrentS->child_begin(), CurrentS->child_end());` add any nullptr into `ToProcess` in this testing context?

I think it is possible, but I could not reproduce this case with the shown code. 

https://github.com/llvm/llvm-project/pull/101031