PM / sleep: Drop unused `info' variable

Commit 32e8d689 (PM / sleep: trace_device_pm_callback coverage in
dpm_prepare/complete) removed all users of this variable but forgot to
remove the variable itself.

Signed-off-by: Thierry Reding <treding@nvidia.com>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>