Corrected a PostgreSQL SELECT statement. (#9460)

At least in my Postgresql 9.6.5, the old syntax of capitalized queries doesn't work (Linux).<br>
Running with UTF-8 as standard encoding and the ` notations didn't work either, so removed those.

pkg/services/sqlstore/migrator/postgres_dialect.go

@@ -104,7 +104,7 @@ func (db *Postgres) SqlType(c *Column) string {
 
 func (db *Postgres) TableCheckSql(tableName string) (string, []interface{}) {
 	args := []interface{}{"grafana", tableName}
-	sql := "SELECT `TABLE_NAME` from `INFORMATION_SCHEMA`.`TABLES` WHERE `TABLE_SCHEMA`=? and `TABLE_NAME`=?"
+	sql := "SELECT table_name FROM information_schema.tables WHERE table_schema=? and table_name=?"
 	return sql, args
 }