tools/power/x86/intel-speed-select: Remove isst_get_pbf_info_complete

isst_get_pbf_info_complete does nothing but just free the core_mask.
Remove the function and do free core_mask directly and free core mask in
the caller.

No functional changes are expected.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
[srinivas.pandruvada@linux.intel.com: changelog edits]
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>