kde5.eclass: don't make kde-plasma/polkit-kde-agent block kde-apps/kde4-l10n.
As sys-auth/polkit-kde-agent was never part of KDE SC 4, it's translations
were never installed by kde-l10n, so there's no need to have a blocker.
In addition to not being required, the blocker was encountered by all
KDE 4 users who had kde-apps/kde4-l10n installed and tried to install a
package that requires kde-frameworks/kauth (which pulls in kde-plasma/polkit-kde-agent).
Resolving the blocker manually by enabling the minimal USE flag on kde-apps/kde4-l10n
caused Plasma 4 translations to be removed.