Remove redundant enabled flag and disableInterceptor method from CacheWriter/Loader Interceptors

Currently both the CacheWriter and CacheLoader interceptors utilise a method disableInterceptor which sets a boolean flag to false. This method is only every called by PersistenceManagerImpl when removing the interceptor, at which point it should not be possible for any of the removed interceptor's methods to be executed. Therefore we should be able to remove this flag, associated checks and the disableInterceptor method without issue.