removed the private attribute pluginID from ElggPlugin

the private attribute pluginID of an ElggPlugin only got filled when creating a new plugin, not if you get the plugin from the database (with guid).

This resulted in some weird behaviour in the functions:
->activate
->deactivate
->set
as these functions trigger events/hooks with ->pluginID to help in the events/hooks. But if this is empty you can't check on it and you had to use entity->getID().

this problem probably is left over from migrating from 1.7 to 1.8.

I have tested this change and everything still works fine