icinga: Removal of icinga addon

icinga: Removal of icinga addon

- As discussed in the Dev conf call on 2024-Jan-08
- The 1.x version of Icinga has been EOL since 2018
- The 2.x version would require a complete new configuration approach as the settings
   and options are completely different to 1.x and so would be a start from scratch.
- removal of icinga from make.sh file
- removal of lfs file
- removal of rootfile
- removal of configuration file
- removal of backup includes file

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Acked-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>