Remove implicitHeight from cards in main page

Summary:
Card now has a correct default height.

Depends on D29127

Test Plan: Main page cards display correctly, both in single and double column mode.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Plasma, Kirigami

Differential Revision: https://phabricator.kde.org/D29129