Remove add() and update() from ISearchIndexManager

Remove add() and update() from ISearchIndexManager

These methods are long-deprecated, remove them from the interface to not force
search engine authors to implement these.

It is expected that code consuming the search API uses the `SearchIndexManager`
class instead of directly accessing a specific `*SearchIndexManager`. The
`SearchIndexManager` only uses `->set()` on the underlying actual
`*SearchIndexManager`. Thus no compatibility break is expected.