kcms/energy: Install using kinfocenter_add_kcm and remove unused metadata

Oliver Beardrequested to merge
work/olib/energy-kcm-infocenter into master

Using kinfocenter_add_kcm ensures that the KCM is installed to the right location to be detected as an Info Center KCM by the systemsettings runner, fixing an issue where the KCM was opened via kcmshell rather than Info Center.

X-KDE-ParentApp is not used in any other kinfocenter KCM metadata and is ultimately unused.

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