Deprecate Kirigami.Label

Summary:
QQC2 Styles style labels themselves, we don't to do this via a
subclass.

This is a relic from Kirigami 1 where labels were not themed, and whilst
it makes some sense for source compatibility, encouraging its use does
not; Both the gallery and systemsettings already has a weird mix of both going on.

This patch also adjusts documentation that talks about Plasma styles;
which I think is a copy-paste error.

Test Plan:
Ran gallery and system settings
Looks the same

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: mart, plasma-devel

Tags: Plasma, Kirigami

Differential Revision: https://phabricator.kde.org/D8131