Fix memory leak as reported by KSPCF

[uriyuzhui]

This should fix the KSPCF complain of RA memory leak that looks like

[LOG 18:15:46.868] [KSPCF:MemoryLeaks] A destroyed RealAntennas:RealAntennasUI instance is owning a onGameSceneLoadRequested GameEvents callback. No action has been taken, but unless this mod is relying on this pattern, this is likely a memory leak.

Same as #83
Briefly tested with a minimum KSP 1.12.3 install, the memory leak complain is gone.