Cleanup: Remove unused modifiers callback

Was only used by subsurf in the past years, it is unlikely
other modifiers will every need this any time soon.

source/blender/modifiers/intern/MOD_multires.c

@@ -265,7 +265,6 @@ ModifierTypeInfo modifierType_Multires = {
 	/* deformVertsEM_DM */  NULL,
 	/* deformMatricesEM_DM*/NULL,
 	/* applyModifier_DM */  NULL,
-	/* applyModifierEM_DM */NULL,
 
 	/* deformVerts */       NULL,
 	/* deformMatrices */    NULL,
@@ -276,7 +275,6 @@ ModifierTypeInfo modifierType_Multires = {
 #else
 	/* applyModifier */     applyModifier,
 #endif
-	/* applyModifierEM */   NULL,
 
 	/* initData */          initData,
 	/* requiredDataMask */  NULL,