[Concurrency] Task: Remove concurrent debugger

(copied from https://github.com/gemoc/gemoc-studio/issues/46)

Currently, we have two separate debuggers:
- sequential: `org.gemoc.execution.sequential.javaengine.ui.debug.GenericSequentialModelDebugger`
- concurrent: `org.gemoc.execution.concurrent.ccsljavaengine.ui.debug.GemocModelDebugger`

However, not only a lot of code is identical between both, but the concurrent debugger does very little concurrency-specific things (even less since some of these things are currently broken).

After discussion, we decided to remove the concurrent debugger and to only provide sequential debugging for now, even in the concurrent case. This means that the sequential debugger msut be adapted to be handle as well as it can (ie. not very well) concurrency, as written here: #32