CI: Remove optimistic continuation of disk image creation for macOS

The situation of the mount point being unmounted after an unsuccessful
detach attempt leads to an unrecoverable situation. Instead of
optimistically continuing, the script has to fail.

.github/scripts/utils.zsh/create_diskimage

@@ -26,14 +26,6 @@ safe_hdiutil() {
   local -r -a _backoff=(2 5 10 15 30)
 
   for i ({1..5}) {
-    if [[ ${1} == detach ]] {
-      if ! [[ -d ${@[-1]} ]] {
-        log_warning "Volume at mountpoint ${@[-1]} is not mounted anymore. Continuing."
-        _status=0
-        break
-      }
-    }
-
     hdiutil ${@} && _status=0 || _status=1
 
     if (( _status )) {