[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.

llvm-svn: 198709