Safe to remove piezo-buzzer from this PCB?
You can completely remove it without any substitution. The Transistors are fully biased in the system by resistors already.

In fact, they have slightly over-engineered it, if you ask me, but these days I'd say I'm pleasantly surprised by such things.

You/it should not even notice the removal by a change in power required for the buzzer circuit to any noticeable extend.