powercap: DTPM: Drop unused local variable from init_dtpm()

The dtpm_descr variable in init_dtpm() is not used after commit
f751db8a ("powercap/drivers/dtpm: Disable DTPM at boot time"),
so drop it.

Fixes: f751db8a ("powercap/drivers/dtpm: Disable DTPM at boot time")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>