delete prep/Linux-s390/305_include_s390_tools.sh

[pcahyna]

the whole check in usr/share/rear/prep/Linux-s390/305_include_s390_tools.sh should now be deleted, it is useless and potentially harmful (any failure during checking /boot will result in all s390 tools omitted from the recovery image, with disastrous results).
Actually this whole file can then be deleted and the PROG+= lines moved to usr/share/rear/conf/Linux-s390x.conf or whatever is appropriate for s390 (this file got deleted in cba3590 without explanation).

Originally posted by @pcahyna in #3136 (comment)

[github-actions]

Stale issue message

[lzaoral]

@jsmeix Could you please reassign this issue to me? Thank you!

[jsmeix]

@lzaoral
my pleasure!