cpupower tool: remove use of undefined variables from the clean target of the top makefile

UTIL_BINS and IDLE_OBJS variables are not defined at all, so
there's no need to remove their content from the 'clean' target.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>