Drop busname unit type

Since busname units are only useful with kdbus, they weren't actively
used. This was dead code, only compile-tested. If busname units are
ever added back, it'll be cleaner to start from scratch (possibly reverting
parts of this patch).

shell-completion/zsh/_sd_unit_files

@@ -5,5 +5,5 @@ _sd_unit_files() {
     files=( '*:files:->files' )
 
     _description files expl 'unit file'
-    _files "$expl[@]" -g '*.(automount|busname|device|mount|path|service|socket|swap|target|timer)'
+    _files "$expl[@]" -g '*.(automount|device|mount|path|service|socket|swap|target|timer)'
 }