Revert "Postgres: Switch the datasource plugin from lib/pq to pgx (#103961)" (#106270)

This reverts commit 1e383b0c1e98734ad4bf974f0435512d10c33246.

pkg/tsdb/grafana-postgresql-datasource/sqleng/sql_engine.go

@@ -19,7 +19,6 @@ import (
 	"github.com/grafana/grafana-plugin-sdk-go/backend/log"
 	"github.com/grafana/grafana-plugin-sdk-go/data"
 	"github.com/grafana/grafana-plugin-sdk-go/data/sqlutil"
-	"github.com/jackc/pgx/v5/pgxpool"
 )
 
 // MetaKeyExecutedQueryString is the key where the executed query should get stored
@@ -89,7 +88,6 @@ type DataSourceHandler struct {
 	dsInfo                 DataSourceInfo
 	rowLimit               int64
 	userError              string
-	pool                   *pgxpool.Pool
 }
 
 type QueryJson struct {
@@ -491,7 +489,6 @@ type dataQueryModel struct {
 	Interval          time.Duration
 	columnNames       []string
 	columnTypes       []*sql.ColumnType
-	columnTypesPGX    []string
 	timeIndex         int
 	timeEndIndex      int
 	metricIndex       int