Revert "[LLVM][Casting.h] Add trivial self-cast"

Revert "[LLVM][Casting.h] Add trivial self-cast"

This reverts commit 0809f63826d36c89574d6ac056ebf46a4b6f29ff.  The patch appears not to have included corresponding isa<Ty> support.

This was revealed when reintroducing the required isa<Ty> asserts in cast<Ty>.  See https://discourse.llvm.org/t/cast-x-is-broken-implications-and-proposal-to-address/63033 for context.

Here's the template instantiation error:
In file included from /home/preames/llvm-repo/llvm-project/llvm/unittests/Support/Casting.cpp:9:
/home/preames/llvm-repo/llvm-project/llvm/include/llvm/Support/Casting.h: In instantiation of ‘static bool llvm::isa_impl<To, From, Enabler>::doit(const From&) [with To = llvm::bar*; From = llvm::bar; Enabler = void]’:
/home/preames/llvm-repo/llvm-project/llvm/include/llvm/Support/Casting.h:110:36:   required from ‘static bool llvm::isa_impl_cl<To, const From*>::doit(const From*) [with To = llvm::bar*; From = llvm::bar]’
/home/preames/llvm-repo/llvm-project/llvm/include/llvm/Support/Casting.h:137:41:   required from ‘static bool llvm::isa_impl_wrap<To, FromTy, FromTy>::doit(const FromTy&) [with To = llvm::bar*; FromTy = const llvm::bar*]’
/home/preames/llvm-repo/llvm-project/llvm/include/llvm/Support/Casting.h:129:13:   required from ‘static bool llvm::isa_impl_wrap<To, From, SimpleFrom>::doit(const From&) [with To = llvm::bar*; From = const llvm::bar* const; SimpleFrom = const llvm::bar*]’
/home/preames/llvm-repo/llvm-project/llvm/include/llvm/Support/Casting.h:263:62:   required from ‘static bool llvm::CastIsPossible<To, From, Enable>::isPossible(const From&) [with To = llvm::bar*; From = const llvm::bar*; Enable = void]’
/home/preames/llvm-repo/llvm-project/llvm/include/llvm/Support/Casting.h:517:38:   required from ‘static bool llvm::CastInfo<To, From, typename std::enable_if<(! llvm::is_simple_type<From>::value), void>::type>::isPossible(From&) [with To = llvm::bar*; From = llvm::bar* const]’
/home/preames/llvm-repo/llvm-project/llvm/include/llvm/Support/Casting.h:556:46:   required from ‘bool llvm::isa(const From&) [with To = llvm::bar*; From = llvm::bar*]’
/home/preames/llvm-repo/llvm-project/llvm/include/llvm/Support/Casting.h:585:3:   required from ‘decltype(auto) llvm::cast(From*) [with To = llvm::bar*; From = llvm::bar]’
/home/preames/llvm-repo/llvm-project/llvm/unittests/Support/Casting.cpp:181:27:   required from here
/home/preames/llvm-repo/llvm-project/llvm/include/llvm/Support/Casting.h:64:64: error: ‘classof’ is not a member of ‘llvm::bar*’
   64 |   static inline bool doit(const From &Val) { return To::classof(&Val); }