[R] Simplify code for handling summarise() with no aggregations

[asfimport]

Check whether ARROW-15609 enables us to remove code from {}[query-engine.R|https://github.com/apache/arrow/blob/master/r/R/query-engine.R]{}.

Reporter: Ian Cook / @ianmcook

Related issues:

• [C++][Compute] Support hash_aggregate with only keys (depends upon)

_{Note: This issue was originally created as ARROW-15719. Please see the migration documentation for further details.}

[github-actions]

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