Fix Syncer Memory Leak

[MojamojaK]

Fix for #6337 .
Any reason to commenting out L1395 in the first place?
https://github.com/IntelRealSense/librealsense/blob/bc64e52c9e128aa82cc9f25a14283bbe2bcf8828/src/sensor.cpp#L1395

[dorodnic]

Hi @MojamojaK
Thank you very much for pointing out the mistake.
I was suspecting that the callback was being leaked, but didn't think to check callback registration code :/
I suspect this was an unintentional change that fall through inside much larger refactoring (7fc0f74)

[MojamojaK]

Great!
I totally agree with it being an unintentional change.
I have tested the fix on my setup, and it seems to be working fine without any memory leaks solely due to the syncer.

[MojamojaK]

Any plans for bumping this?

[ev-mp]

@MojamojaK , we'll merge it now.
Thank again for the fix!