cpupower tools: Remove brace expansion from clean target

The clean targets from the cpupower tools' Makefiles use brace expansion to
remove some generated files. However, the default shells on many systems do
not support this feature resulting in some generated files not being removed
by clean.

Signed-off-by: Palmer Cox <p@lmercox.com>
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>