Remove the `killme` script command

Isn't used in any scripts and probably doesn't work correctly anyway due
to improper SHOW_FLAG_KILLED handling.
Remove entity show flag reset code in SetEditMode

Despite the name, this function isn't only called in edit mode and this
code makes no sense for normal gameplay.

Besides, the only entity that should exist at this point is the player.
Remove unused TexturedVertex and EE_RTP() call
Move TREATZONE_RemoveIO(this) to cleanReferences()
Use Entity::destroy() in replaceme script command
Set SHOW_FLAG_DESTROYED in Entity::destroy()

Previously it set SHOW_FLAG_KILLED if the entity wasn't created by a
script and *deleted* otherwise. The only reason entities not loaded by
scripts aren't immediately destroyed is that they need to be written to
the save file so that we can remember their destruction. This is needed
because they are still referenced by the level file and because we don't
want to reuse their instance number as that may be associated with a
special script that should not be used for random objects of the class.

The KILLED flag will however not prevent the entities from receiving
script events and will cause them to be needlessly loaded again from
save file, bloating the runtime entity list.
Don't leak items in the `inventory destroy` cmd

The `inventory create` command already correctly clears the inventory,
so there is no point in the `inventory destroy` command doing it
differently.
Use Entity::destroy() in ARX_INTERACTIVE_DestroyIO
Use Entity::destroy() when combining item stacks
Remove ARX_SCRIPT_Timer_Clear_By_IO()

Duplicate of ARX_SCRIPT_Timer_Clear_For_IO()
Merge more cleanup code into Entity::destroy()
Remove duplicated IO_UnlinkAllLinkedObjects() code
Use Entity::destroy() in ARX_PLAYER_ManageTorch()
Move ARX_INTERACTIVE_DestroyIO() in Entity, rename
Remove redundant IO_UnlinkAllLinkedObjects() call

Already called via ARX_INTERACTIVE_DestroyDynamicInfo().
find-script-command: Use lookbehind and lookahead

grep was choking if the matched string included a cariiage return and
displayed empty lines.
Use Entity::destryOne() for the `eatme` script cmd

ARX_DAMAGES_ForceDeath() doesn't make sense as this was only called from
items, which have no death event.
However, it's not always the last call so use delay the entity
destruction to allow the remaining script to be executed.
Don't load entities with show == SHOW_FLAG_KILLED

SHOW_FLAG_KILLED isn't used anymore. Before it was used like
SHOW_FLAG_DESTROYED, but didn't completely clean up the entity.
Don't save destroyed entities in the level index

We won't load them anyway, so adding them to the index is pointless.
Add warning when trying to load IO_NOSAVE entities