Cleanup: Remove modifier type hair callback

This is similar to e032ca2e25 which removed the
callback for volumes. Now that we have geometry sets, there is
no need to define a callback for every data type, and this wasn't
used. Procedural curves/hair editing will use nodes rather than new
modifier types anyway.

source/blender/modifiers/intern/MOD_datatransfer.c

@@ -484,7 +484,6 @@ ModifierTypeInfo modifierType_DataTransfer = {
     /* deformVertsEM */ NULL,
     /* deformMatricesEM */ NULL,
     /* modifyMesh */ modifyMesh,
-    /* modifyHair */ NULL,
     /* modifyGeometrySet */ NULL,
 
     /* initData */ initData,