Remove init_worktree from sync config

It was currently unused and only confuses. The initialization of
worktrees can currently only be controlled via the `--init-worktree`
command line switch. This is unfortunate, but it's the only was to
handle it right now. Changing it would mean a restructure of the code,
mainly the `tree::sync_trees` function.

src/config.rs

@@ -52,7 +52,6 @@ pub struct ConfigProvider {
     pub api_url: Option<String>,
 
     pub worktree: Option<bool>,
-    pub init_worktree: Option<bool>,
 
     pub remote_name: Option<String>,
 }