CLEANUP: Remove appsession documentation

CLEANUP: Remove appsession documentation

I was about to partly revert 294d0f08b3d100fcae0e71c26d4f9f93d26e3569,
because there were no 'X' for 'appsession' in the keyword matrix until
I checked the blame, realizing that the feature does not exist any more.

Clearly the documentation is confusing here, the removal note is only
listed *below* the old documentation and the supported sections still
show 'backend' and 'listen'.

It's been 3.5 years and 4 releases (1.6, 1.7, 1.8 and 1.9), I guess
this can be removed from the documentation of future versions.