Remove skip test for missing json type

All supported versions of PostgreSQL now have json type.

example_json_test.go

@@ -2,6 +2,7 @@ package pgx_test
 
 import (
 	"fmt"
+
 	"github.com/jackc/pgx"
 )
 
@@ -12,13 +13,6 @@ func Example_JSON() {
 		return
 	}
 
-	if _, ok := conn.PgTypes[pgx.JSONOID]; !ok {
-		// No JSON type -- must be running against very old PostgreSQL
-		// Pretend it works
-		fmt.Println("John", 42)
-		return
-	}
-
 	type person struct {
 		Name string `json:"name"`
 		Age  int    `json:"age"`