KRunner: port away from deprecated KF5 API

Summary:
Plasma::Package has been deprecated since 5.28, but it's used
in the public API for AbstractRunner.

Removed the underlying member variable which is never set
(since apol's commit 8d77a33b6eae26).

Once krunner is ported to the new deprecation macros, I guess
the package() method will need something like

#if KRUNNER_ENABLE_DEPRECATED_SINCE(5, 28)

Technically the method wasn't marked as deprecated between 5.28 and 5.64
but the class it returned, was, so that's pretty much the same.

Test Plan: Builds

Reviewers: mart, apol, dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24979