[IR] Remove Synthetic Profile Support from Function
Synthetic profiles are not generated anywhere and support is very
sporadic across the code base. They are slated to be removed, so remove
support for them from Function member functions.
A future PR will clean up the ProfileCount abstraction that is now no
longer necessary.
Reviewers: teresajohnson, david-xl, mtrofin
Pull Request: https://github.com/llvm/llvm-project/pull/204768
