Remove kpluginindex.json caching
As discussed in https://phabricator.kde.org/T14090 caching mechanism is based in deprecated Qt API. Based on the benchmarks the performance implications are not critical.
Also the usages in the KCMs will get optimized to not reload all of the installed packages whenever anything in KNewStuff is changed, see plasma/plasma-desktop!410 (4afd5a39) and https://bugs.kde.org/show_bug.cgi?id=431568.