Remove pointless data copy in lockscreen delegate

Summary: Not sure what I was thinking when I wrote it.

Test Plan:
Grepped m is not used in file.
Grepped it's not used in any other users of the delegate

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D7473