kf5-base-applications: Drop khelpcenter

KHelpCenter isn't needed for a minimal desktop build. When khelpcenter
is not installed, the various "Help" buttons in apps will simply open
the online version of the documentation in the default browser.
2 jobs for no-khelpcenter in 1 minute and 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #55566
smoke-test

00:01:52

passed #55565
unit-test

00:00:37