[clang-tools-extra] [clang-tidy][NFC] Remove unnecessary nullptr check on cast subexpr (PR #85473)

[Mike Rice via cfe-commits]

mikerice1969 wrote:

I mentioned the assert just to make the point that setSubExpr is written so it doesn't return a nullptr. So it is not expected and the deference is ok. 

I went ahead with this change but feel free to update if you like. But adding a return when nullptr is seen here would confuse the code and static verifiers will complain that the return can never happen.

https://github.com/llvm/llvm-project/pull/85473