GUACAMOLE-935: Do not assign SVC plugin context as it may be automatically freed.

FreeRDP version 2.0.0-rc0 and older will automatically free whatever
entry_points->pInterface is set to when the channel plugin is unloaded.
This doesn't happen in later versions, but will result in a double-free
upon disconnect for 2.0.0-rc0 in our case. As we don't need pInterface,
we can safely set this to NULL and avoid the issue entirely.
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