Incomplete removal of timer voice channel

[Intery]

Issue

Unsetting the voice channel using ,ptconfig voice_channel None does not remove the voice channel from the timer (e.g. the timer continues giving voice alerts).

Proposed Solution

Ref: https://github.com/Intery/PomoBot/blob/rewrite/bot/Timer/core.py#L431 The Timer.load() method, called by the timer config setter, should reset the voice channel when the voice channel id is None.