Remove GroupBy.field_name from BanyanDB MeasureQuery request building

[currenjin]

What this PR does

Phase 1 of the staged removal described in #13779.

QueryRequest.GroupBy.field_name is not used by BanyanDB's query execution path. BanyanDB groups by tag_projection, not by field_name. Sending this field is misleading and should be removed.

This PR removes the single groupByBuilder.setFieldName(this.aggregation.fieldName) call in MeasureQuery.build(), so the GroupBy message now only carries tag_projection.

The Aggregation.field_name and Top.field_name fields are unrelated (they specify which field to aggregate on / sort by) and are intentionally left unchanged.

Subsequent steps

• Phase 2: Remove GroupBy.field_name from the client proto repo.
• Phase 3: Remove the field from BanyanDB server.

Related

Closes #13779 (Phase 1)