Config drive: delete deprecated config var config_drive_tempdir

Commit 6e15d787fffcef8c4299fbaf1d5c715af1a34fa2 removed the use
of this configuration variable.

Change-Id: Ifd5baa8b7d579b0959cdb57b1335380a4129d652
Merge "Config drive: delete deprecated config var config_drive_tempdir"