Fixes gravity generator hard del (#85390)

## About The Pull Request

Tin.

![image](https://github.com/user-attachments/assets/4a64746a-1886-4148-9ac6-791d31046013)

## Why It's Good For The Game

Less hard dels is a good thing generally

## Changelog

Not player facing

code/modules/power/gravitygenerator.dm

@@ -78,6 +78,7 @@ GLOBAL_LIST_EMPTY(gravity_generators)
 /obj/machinery/gravity_generator/part/Destroy()
 	atom_break()
 	if(main_part)
+		main_part.generator_parts -= src
 		UnregisterSignal(main_part, COMSIG_ATOM_UPDATED_ICON)
 		main_part = null
 	return ..()