The ctor was added with d1e47b14 "enhance
useuniqueptr loplugin" apparently because it was needed and the implicitly
declared one stareted to be defined as deleted, while the assignment op was
left implicitly defined as deleted (presumably because it wasn't needed anyway).

Adding a non-deleted definition of the assignment op probably is a good move
towards consistency, and was assumed it could help avoid new Clang trunk
-Werror,-Wdefaulted-function-deleted in derived ConfigItem
(include/unotools/configitem.hxx), but which appears not to be the case.

And ConfigurationBroadcaster can't easily switch from copy to move semantics, as
SdOptionsLayoutItem::Clone needs SdOptionsLayoutItem copy ctor needs
SdOptionsLayout copy ctor needs SdOptionsGeneric copy ctor needs
SdOptionsItem copy ctor needs utl::ConfigItem copy ctor needs
utl::ConfigurationBroadcaster copy ctor.

Change-Id: Ie1f4e72eacb9b6b776103c4cf04a959acfa6648f
Reviewed-on: https://gerrit.libreoffice.org/61208
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>