touch: Remove deprecated ApplicationHeader

Summary:
Resolves the following warning:

qml: Warning: ApplicationHeader is deprecated, remove and use the automatic internal toolbar instead.

Test Plan:
Tested on both desktop and Plasma Mobile.
Looks a bit different on mobile because the "automatic internal toolbar" is smaller than the ApplicationHeader but it even looks better that way.

Reviewers: svuorela

Reviewed By: svuorela

Differential Revision: https://phabricator.kde.org/D26340