revert iot dig_for_message

[jamesmf]

Description

fixes #2635
#2635

Reverts the code that assumes that the CommonIoTSkill asks for other skills to register instead of individual skills calling register_words of their own accord.

Discussion with author of the change being reverted ( @JarbasAI ) here:
https://chat.mycroft.ai/community/pl/w66y18fgj7ygt8u3j6pdsxsofc

How to test

Try to install a skill that extends CommonIotSkill like the HomeAssistant branch

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[devops-mycroft]

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[devops-mycroft]

Voight Kampff Integration Test Succeeded (Results)

[forslund]

Great catch, this change looks good.