Revert "Force a new instance of cinnamon-background-worker on screen changes.."

This reverts commit e08cb4b65346e740a34aab6dc889a69ff67a416b.
Revert "Externalized background manager to a new "cinnamon-background-manager" process, which launches "cinnamon-background-worker"."

This reverts commit 6fab8e847232671b8f791bfc300314045bdb77e0.