Remove assertion for disconnect

[DivineDominion]

https://github.com/adamnemecek/WebMIDIKit/blob/6f3506937c141f04edaaa182502984d7baaa2e26/Sources/WebMIDIKit/MIDIPortMap.swift#L70

This assertion fires when you unplug a MIDI device of which a port is used.

I think removing this cause for runtime exceptions in DEBUG builds is a good fit (because the assumption that the device is still connected isn't always true!) and cannot gracefully be handled at all.