PM / Clocks: Remove redundant NULL checks before kfree()

Since kfree() checks it its argument is not NULL, it is not necessary
to duplicate this check in __pm_clk_remove().

[rjw: Added the changelog.]

Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>