Remove commander plugin (broken)

The plugin is not able to load:

```
** (gedit:22113): WARNING **: 06:52:01.641: Error importing plugin 'commander':
Traceback (most recent call last):
  File "/usr/lib64/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "[...]/gedit/plugins/commander/__init__.py", line 34, in <module>
    from appactivatable import CommanderAppActivatable
  File "[...]/gedit/plugins/commander/appactivatable.py", line 24, in <module>
    import commander.commands as commands
  File "[...]/gedit/plugins/commander/commands/__init__.py", line 26, in <module>
    from . import module, method, result, exceptions, metamodule, completion
  File "[...]/gedit/plugins/commander/commands/module.py", line 25, in <module>
    import commands.exceptions as exceptions
  File "[...]/gedit/plugins/commander/commands/__init__.py", line 26, in <module>
    from . import module, method, result, exceptions, metamodule, completion
  File "[...]/gedit/plugins/commander/commands/completion.py", line 22, in <module>
    import commander.commands as commands
AttributeError: module 'commander' has no attribute 'commands'

(gedit:22113): libpeas-WARNING **: 06:52:01.642: Error loading plugin 'commander'
```

It's maybe easy to fix, I really don't know.

But nobody reported the bug. The plugin is aimed at advanced users,
while gedit primarily targets beginners.

If somebody is interested to revive this plugin, it can be put in its
own repo.