Summary:
Commit 9ec8eee2 had removed the custom
actions prev_error & next_error in favour of some global actions with
global shortcut, which would dynamically decide which result tool view to
apply the action (e.g. by shortcut) on. Two things were forgotten:

a) the plugin still installed a ui.rc file for those actions

b) The OutputWidget still tried to get the two actions by their id from
the plugin, to insert it into its own UI. Later the commit
fc8d34f0 also added generic actions to
the widget. As a result of the failing action fetching two warnings
QWidget::insertAction: Attempt to insert null action
were seen on the log.

This commit fixes both by removing the no longer useful code.

Test Plan:
Output widget still works WRT item navigation, both from main menu and from
toolview actions.

Reviewers: #kdevelop, kfunk

Reviewed By: #kdevelop, kfunk

Subscribers: kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D15432