refactor: remove git:unlock command

This is no longer necessary with improvements elsewhere in Dokku.

plugins/git/help-functions

@@ -38,7 +38,6 @@ fn-help-content() {
     git:public-key, Outputs the dokku public deploy key
     git:report [<app>] [<flag>], Displays a git report for one or more apps
     git:set <app> <property> (<value>), Set or clear a git property for an app
-    git:unlock <app> [--force], Removes previous git clone folder for new deployment 
     git:status <app>, show the working tree status for an app
 help_content
 }