[MIRROR] Abductors can no longer be converted by conversion antags (#29310)

* Abductors can no longer be converted by conversion antags (#84766)

## About The Pull Request

Basically what the title says. Abductors cannot be converted by
bloodbrother/revs/cult. They will NOT show up as having a mindshield,
however.
## Why It's Good For The Game

Abductors are, by design, incredibly strong. You are not meant to be
able to win against them, and this is reflected policy-wise by the fact
that they're a restricted antagonist. However, this is still a problem
when conversion antags get involved. If an abductor becomes a cultist or
a revolutionary, then suddenly every other player who isn't also
converted has to deal with an abductor without the normal restrictions.
It's not fun to play against and in general just shouldn't happen.
## Changelog
🆑
balance: Abductors (the antag, not the species) can no longer be
converted by any antagonist.
/🆑

* Abductors can no longer be converted by conversion antags

---------

Co-authored-by: GPeckman <21979502+GPeckman@users.noreply.github.com>

code/datums/mind/_mind.dm

@@ -64,7 +64,6 @@
 	///If this mind's master is another mob (i.e. adamantine golems). Weakref of a /living.
 	var/datum/weakref/enslaved_to
 
-	var/unconvertable = FALSE
 	var/late_joiner = FALSE
 	/// has this mind ever been an AI
 	var/has_ever_been_ai = FALSE