build: fix unused variable after last patch

The previous commit (2cb0899e) left a dead variable behind.

* src/libxl/libxl_driver.c (libxlClose): Drop dead variable.
(cherry picked from commit b8e6021e)

src/libxl/libxl_driver.c

@@ -1079,8 +1079,6 @@ libxlOpen(virConnectPtr conn,
 static int
 libxlClose(virConnectPtr conn ATTRIBUTE_UNUSED)
 {
-    libxlDriverPrivatePtr driver = conn->privateData;
-
     conn->privateData = NULL;
     return 0;
 }