Remove designer plugin generation for KF5WebKit

Summary:
KF5WebKit has been tagged as "Porting Aids", so as normal KF tier3
oroduct kdesignerplugin shall no longer depend on it, even optionally.

KF5WebKit at the same time gets modified to generate a designer plugin
during its own build, to be enabled with BUILD_DESIGNERPLUGIN=ON cmake flag.

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22582