mender-convert-modify: remove superfluous 'rm' in empty dir cleanup

The 'rmdir' call should be enough, and just because there is an
empty dir in the boot part, does not mean that it exists in
rootfs/boot, so just remove this.

Changelog: Fix error when removing empty directories in rootfs/boot

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>

mender-convert-modify

@@ -207,7 +207,6 @@ EOF
   sudo rm -rf work/boot/NvVars
   for empty_dir in $(cd work/boot; find . -maxdepth 1 -type d -empty); do
     sudo rmdir work/boot/$empty_dir
-    sudo rm work/rootfs/boot/$empty_dir
   done
 
   log_info "Installing GRUB"