Bug 35428: Remove temporary files for gulp po tasks

Bug 35428: Remove temporary files for gulp po tasks

gulp po:update will generate 1 /tmp/koha-* directory per task.

We need to remove them when the script does not longer need them.

Test plan:
0. Do not apply this patch
1. ls -ltr /tmp
2. gulp po:update --lang es-ES
3. ls -ltr /tmp
=> Notice new koha-* directories
Repeat 2, 3
4. Apply this patch and try again
=> No new directories when the job is done

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>