[krunner] Avoid a uneeded pointless config parse

Summary:
We are krunner, our default config will be krunnerrc, we can load the
instance we've already loaded in the QPT instead of doing it again.

Reviewers: Plasma, apol, broulik

Reviewed By: Plasma, apol, broulik

Subscribers: apol, plasma-devel

CCBUG: 416145
Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28368