Alternative to diode
The 0.15V drop is acceptable if the RTC is specified to run at 2.8V, or make it 2.7V. As long as these Lithium Coin cells are not completely depleted, they stay above 3V (fresh from the Box: 3.2V) quite well.

If there is a dedicated resistor for trickle charging that is not used in the circuit otherwise, just remove it, though.