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