Remove unused ansible variables from documentation and fix example

[arjanoosting]

These constants are not used. This is now handled by the i2_custom_const variable Fixes #35

[mkayontour]

As I wrote in my answer at #35 we should set all constants as default value.

[martialblog]

Hi, This role is no longer maintained. See: https://github.com/Icinga/ansible-collection-icinga