Tweaked diona and fixed slimepeople eyes (#27950)

Removed the human eyes layered on top of dionea, and removed the option to change the color of diona and slimepeople eyes.

code/modules/client/preference/preferences.dm

@@ -299,7 +299,7 @@ GLOBAL_LIST_INIT(special_role_times, list(
 				active_character.f_style = "Shaved"
 
 
-			if(!(S.bodyflags & ALL_RPARTS))
+			if(!(S.bodyflags & ALL_RPARTS) && (S.eyes != "blank_eyes") && !(S.bodyflags & NO_EYES))
 				dat += "<b>Eyes:</b> "
 				dat += "<a href='byond://?_src_=prefs;preference=eyes;task=input'>Color</a> [color_square(active_character.e_colour)]<br>"
 

code/modules/mob/living/carbon/human/species/diona_species.dm

@@ -17,6 +17,7 @@
 	As a gestalt entity, each nymph possesses an individual personality, yet they communicate collectively. \
 	Consequently, Diona often speak in a unique blend of first and third person, using 'We' and 'I' to reflect their unified yet multifaceted nature."
 
+	eyes = "blank_eyes"
 	species_traits = list(NO_HAIR)
 	inherent_traits = list(TRAIT_NOGERMS, TRAIT_NODECAY)
 	inherent_biotypes = MOB_ORGANIC | MOB_HUMANOID | MOB_PLANT