[analyzer] Remove unused ARCNotOwnedSymbol retain count return effect.

[analyzer] Remove unused ARCNotOwnedSymbol retain count return effect.

RetainCountChecker has to track returned object values to know if they are
retained or not. Under ARC, even methods that return +1 are tracked by the
system and should be treated as +0. However, this effect behaves exactly
like NotOwned(ObjC), i.e. a generic Objective-C method that actually returns
+0, so we don't need a special case for it.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198709 91177308-0d34-0410-b5e6-96231b3b80d8