test: mutation_query_test: use local data_query() implementation
test: mutation_query_test: use local data_query() implementation

The test only wants to test result size calculation so it doesn't need
the whole `data_query()` logic. Replace the call to `data_query()` with
one to a local alternative which contains just the necessary bits --
invoking `data_querier::consume_page()` with the right result builder.
This allows us get rid of the now otherwise unused `data_query()`.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla