[w3c/manifest] Editorial: remove installability signals (#966)

This change (choose at least one, delete ones that don't apply):

* Makes editorial changes (changes informative sections, or changes normative sections without changing behavior)

Commit message:

The installability signals didn't really catch on (at least, in the overly broad way currently outlined in the spec). So, for spec brevity, I'm proposing we remove them as they are in an implementation detail.


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You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/966

-- Commit Summary --

  * Editorial: remove installability signals

-- File Changes --

    M index.html (49)

-- Patch Links --

https://github.com/w3c/manifest/pull/966.patch
https://github.com/w3c/manifest/pull/966.diff

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