[api] Cleanup `Decls`: remove unused function, move vernac helper.

It seems these two functions don't belong there. We can remove one,
and place the other actually next to whether their semantics are
necessary.

Note that indeed the whole `Decls` file seems a bit suspicious, why we
do we register this information in a separate table instead of in the
main ones in `Lib` ?

At the suggestion of Gaëtan Gilbert we also remove unused function
`is_instance`.

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