Meta: Use rm -rf instead of rmdir in QEMU image build script

If you run `serenity.sh run` with missing fuse2fs and/or fusermount3,
then install the needed packages, and then try to run the script
again, rmdir step will fail with "non-empty directory" error
causing QEMU to not start. The patch avoids this by recursively
removing disk mount directory instead of just unlinking it.

Meta/build-image-qemu.sh

@@ -159,10 +159,8 @@ cleanup() {
             else
                 umount mnt || ( sleep 1 && sync && umount mnt )
             fi
-            rmdir mnt
-        else
-            rm -rf mnt
         fi
+        rm -rf mnt
 
         if [ "$(uname -s)" = "OpenBSD" ]; then
             vnconfig -u "$VND"