Bug 6997: Koha-remove leaves system in inconsistent state

Bug 6997: Koha-remove leaves system in inconsistent state

If you accidentally delete one of the files that koha-remove is supposed to
remove, when koha-remove reaches that point in the script, it will die, leaving
later removal steps undone. This patch fixes the problem by checking for the
existence of each file prior to deleting it, so that short of an actual problem
with removing the file, the script can continue. Note that the fix for bug 6929
is also needed to prevent any problems with stopping Zebra from killing
koha-remove.

Signed-off-by: Dobrica Pavlinusic <dpavlin@rot13.org>