Remove superfluous code for empty KIPI toolbar button

Summary:
In case there are no KIPI plugins installed, Gwenview needs to show the
No Plugin Found entry instead.

Since the corresponding action is already added to mExportActionList
in KIPIInterface::pluginActions, we don't need to add it again.

Test Plan:
Uninstall Discover and kipi-plugins, check that KIPI toolbar
button still shows No Plugin Found.

Reviewers: Gwenview, faridb

Reviewed By: faridb

Differential Revision: https://phabricator.kde.org/D13251