potodo: deprecated --offline option

[mmmarcos]

The progress page is showing an error with potodo:

The --offline option was available in potodo 0.21.2 but is now deprecated in 0.21.3 even though it only changed the patch number :-(

Release notes are not available yet for 0.21.3 but it seems that the option was removed as part of another issue: AFPy/Potodo#129
It should be enough to just remove the option.

Side note: we should probably lock our deps :)