Prepare takes context

Also remove PrepareEx. It's primary usage was for context. Supplying
parameter OIDs is unnecessary when you can type cast in the query SQL.
If it does become necessary or desirable to add options back it can be
added in a backwards compatible way by adding a varargs as last
argument.

stdlib/sql.go

@@ -162,7 +162,7 @@ func (c *Conn) PrepareContext(ctx context.Context, query string) (driver.Stmt, e
 	name := fmt.Sprintf("pgx_%d", c.psCount)
 	c.psCount++
 
-	ps, err := c.conn.PrepareEx(ctx, name, query, nil)
+	ps, err := c.conn.Prepare(ctx, name, query)
 	if err != nil {
 		return nil, err
 	}
@@ -242,7 +242,7 @@ func (c *Conn) QueryContext(ctx context.Context, query string, argsV []driver.Na
 	// TODO - remove hack that creates a new prepared statement for every query -- put in place because of problem preparing empty statement name
 	psname := fmt.Sprintf("stdlibpx%v", &argsV)
 
-	ps, err := c.conn.PrepareEx(ctx, psname, query, nil)
+	ps, err := c.conn.Prepare(ctx, psname, query)
 	if err != nil {
 		// since PrepareEx failed, we didn't actually get to send the values, so
 		// we can safely retry