Fixed #607
Replaced IsValid() check in the CheckIndex function with Holo.ent == NULL or Holo.ent == nil
Centralized cleanup of the hologram's variables in the CallOnRemove that is created with the hologram to prevent people from getting multiple extra holograms by removing entities in a certain order.
Cleaned up the hologram removal functions at @Divran's request(demand)
Merge pull request #608 from LtBrandon/607-hologram-fix

Fix #607. Fixes holograms not lowering the holo count when they're deleted