Remove group by join

[mtoy-googly-moogly]

Found this test for an experimental feature we need to kill.

it('group by join - simple group by', async () => {
    await expect(`
      run: aircraft->{
        group_by: aircraft_models
        aggregate: aircraft_count
      }
  `).malloyResultMatches(expressionModel, {
      aircraft_models_id: 7102802,
      aircraft_count: 58,
    });
  });

  // when structs are referenced in queries, incorporate the
  //  primary key of struct and add the struct as a join to the result.
  getAsQueryField(): QueryFieldStruct {
    if (this.fieldDef.primaryKey === undefined) {
      throw new Error(
        `Joined explores can only be included in queries if a primary key is defined: '${this.getFullOutputName()}' has no primary key`
      );
    }

    const pkField = this.getPrimaryKeyField(this.fieldDef);
    const pkType = pkField.fieldDef.type;
    if (pkType !== 'string' && pkType !== 'number') {
      throw new Error(
        `Unknown Primary key data type for ${pkField.fieldDef.name}`
      );
    }
    const aliasName = getIdentifier(this.fieldDef);
    const pkName = this.fieldDef.primaryKey;
    const fieldDef: FieldDef = {
      type: pkType,
      name: `${aliasName}_id`,
      e: [
        {
          type: 'field',
          // path: pkField.getFullOutputName(),
          path: pkField.getIdentifier(),
        },
      ],
    };
    return new QueryFieldStruct(fieldDef, this, `${aliasName}.${pkName}`);
  }