tools/power turbostat: Remove add_msr_perf_counter_()

As the only caller of add_msr_perf_counter_(), add_msr_perf_counter()
just gives extra debug output on top. There is no need to keep both
functions.

Remove add_msr_perf_counter_() and move all the logic to
add_msr_perf_counter().

No functional change.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>