backend: remove all files early

We encountered a problem with late files removal. Those removals were
clashing with postinst scripts. Postinst script expect to be run on
fully updated system and having leftover files from older version was
not expected. This had effect like removal of files that were generated
by postinst or postrm scripts or uninstalled plugins being loaded to
applications on service restart by postinst.

This just simply reuses existing facility to remove files early during
files merge. Instead of using it to remove files that collide with
directories we use it to remove all abundant files. It is not an ideal
solution but we are unable to drop previous approach because of backward
compatibility with previous journals.
7 jobs for swap-post-cleanuo in 4 minutes and 21 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Test
passed #253740
cppcheck

00:00:28

passed #253741
luacheck

00:00:28

passed #253738
system-tests

00:01:25

passed #253739
system-valgrind-tests

00:02:58

passed #253736
unit-tests

00:00:52

passed #253742
unit-valgrind-tests

00:02:41

failed #253737
unit-valgrind-tests

00:00:50