sandbox: remove global variable of podsandbox controller

Signed-off-by: Abel Feng <fshb1988@gmail.com>

client.go

@@ -721,9 +721,6 @@ func (c *Client) SandboxStore() sandbox.Store {
 // SandboxController returns the underlying sandbox controller client
 func (c *Client) SandboxController(name string) sandbox.Controller {
 	// default sandboxer is shim
-	if len(name) == 0 {
-		name = "shim"
-	}
 	if c.sandboxers != nil {
 		return c.sandboxers[name]
 	}