Dead code cleanup: remove "show_progress_bar" from PortalTask

[prioux]

There is a method "show_progres_bar?" in PortalTask, and it is 'used' in the "show" page of tasks.

Only one task in an external plugin has even implemented that feature, and that one task is no longer used anyway.

1. remove the method
2. remove the code in tasks/show.html.erb

[prioux]

I will do this once the cbrain-plugins-psom repo has fully been updated to remove the descriptors from the old integration folder