If you do not need RTC-only mode and the RTC timer feature, you do not need to include a 32 KHz crystal. The 32 KHz reference can come from the high frequency clock. Leave the RTC_XTALIN/RTC_XTALOUT pins as NC.
Per Advisory 1.0.30 in the device Silicon Errata, VSS_OSC and VSS_RTC should be connected to system ground.
It is preferable to always have bias and dampening resistors that can help tune the crystal later. For more details, see the Clock Specifications section of the device-specific data sheet.