completions/zsh: Hide *instal from all commands

[okdana]

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After my other completion PR was merged, someone contacted me to ask if i could help with this Discourse post. The user is bothered by the fact that the zsh completion function offers one-letter-short aliases like instal and uninstal. I agree that they aren't helpful, as they produce confusing-looking output and force unnecessary disambiguation when the user enters (e.g.) unintab.

These aliases were already hidden in the bash completion function in PR #3052. This change just does the equivalent for the zsh function.

Note for anyone testing this: If you have caching enabled, you may need to delete the cache files (under ~/.zcompcache by default) to see the change right away.

[jonchang]

Thanks for the help @okdana!