flang] Remove remaining uses of llvm::Optional (NFC)

flang] Remove remaining uses of llvm::Optional (NFC)

This patch removes the unused "using" declaration and removes #include
"llvm/ADT/Optional.h".

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716