Stop generating API docs for Citadel(EOL) (#606)

Signed-off-by: Addisu Z. Taddese <[email protected]>

Author: azeey

.github/workflows/nightly-upload.yml

@@ -16,9 +16,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - ubuntu_distribution: focal
-            gazebo_distribution: citadel
-
           - ubuntu_distribution: focal
             gazebo_distribution: fortress