Remove redundant app name from header

Summary: Now that we're using the header for buttons, the app name name starts to make it look a bit crowsed. Also, it's redundant, since the name is listed again right below it. So, let's remove it.

Test Plan:
{F5755196}

Man, Discover is looking really good these days.

Reviewers: apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11364