The crystal (XTAL) is required only if you plan on using the RTC. If you omit the XTAL, then you should ground the 32KIN pin, and the 32KOUT pin should be left unconnected. By omitting the crystal you obviously would not have RTC functionality. Likewise, you will not be able to wake on the RTC. You also lose the ability to use 32KCAL (to clock other chips requiring 32kHz).