doc: don't generate class diagrams
doc: don't generate class diagrams

This is only useful for C++. Maybe we'll restore it once we have C++
bindings done.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

clemensg/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,clemensg/libgpiod,brgl/libgpiod