No LSE connection to STM32. Is it OK?
It should not cause any problems leaving it out. Sometimes a low frequency secondary oscillator like this might be used for more than one peripheral (e.g. alt clock for timer) but in this case I think it is only used for the RTC.
Also, in this case it looks like even if you leave it out you can still clock the RTC from one of the other oscillators (HSE OSC or LSI RC) 

The clock tree diagram in the datasheet seems to confirm this, note the LSE OSC only goes to the RTCCLK mux (with another 2 options noted above for the RTC clock) and nowhere else: