Don't force a check for updates when the applet runs

Summary:
The timer in main.qml has "triggeredOnStart: true" so it already
checks when the applet runs if the condition to check for updates
is true and if so, it calls PkUpdates.checkUpdates.

Previously, if a user configures the applet so that it only checks
for updates weekly, the applet ignores this configuration and
forces a check for updates every time the user logs in the system
(even more than once per day)

Reviewers: jgrulich

Reviewed By: jgrulich

Differential Revision: https://phabricator.kde.org/D20231