Don't export QT_PLUGIN_PATH

Currently we are modifying the plugin path to include
$kdehome/lib/kde5/plugins, the original change says this is to allow
users to load styles and such.

In Plasma 5 $kdehome is .config, which shouldn't contain any plugins, so
this line currently isn't achieving anything.

Apparently instead it was causing problems and various distros are
already patching it out without issue.

BUG: 349371
FIXED-IN: 5.4.0