[ADT] Remove Optional::{has_value,value}, etc (NFC)

This patch removes methods that I deprecated on August, 2022 in
commits b5f8d42e and
eeac9e92.

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