Get rid of -Wdeprecated-copy warning

Summary:
Remove the explicit copy constructor of KItemListStyleOption, since it does not
add any logic to what the implicit copy constructor does, but triggers a compiler
warning if we don't overload operator= too.

Test Plan:
Compiles without -Wdeprecated-copy warning
No behavior changes

Reviewers: elvisangelaccio, Dolphin

Reviewed By: elvisangelaccio, Dolphin

Subscribers: kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D26585