Removed unnecessary _BaseSound._unload_instance function.

[CookStar]

This fixes the following error when unloading the plugin

Output:

[Source.Python]
[SP] Caught an Exception:
Traceback (most recent call last):
  File "..\addons\source-python\packages\source-python\plugins\manager.py", line 437, in _unload_auto_unload_instances
    instance._unload_instance()
  File "..\addons\source-python\packages\source-python\engines\sound.py", line 292, in _unload_instance
    self._downloads._unload_instance()
  File "..\addons\source-python\packages\source-python\stringtables\downloads.py", line 98, in _unload_instance
    _downloadables_list.remove(self)

ValueError: list.remove(x): x not in list