Remove warning about Debian from issue template
Remove warning about Debian from issue template

Debian no longer offers Nikola, neither in the development tree
("Debian unstable") nor in any supported releases.

Fixes #3057

getnikola/nikola,okin/nikola,getnikola/nikola,okin/nikola,getnikola/nikola,getnikola/nikola,okin/nikola,okin/nikola