libtaskmanager: fix launch url for startup info with pathless desktop file

Summary:
Also drop checking if the suffix exists, the code is only reached
because this condition has been checked before

Seen while browsing code, no app known to me which would set a non-absolute
desktop file.

Reviewers: hein, broulik, #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23417