Cleanup: remove unused IDProperty members

Remove:

- IDPropertyTemplate.matrix_or_vector
  Matrix & vector types have been deprecated, this wasn't used.

- IDProperty.saved
  This was added preemptively but never used, replace with a pad so as
  not to hint at a feature that doesn't exist.

source/blender/blenkernel/BKE_idprop.h

@@ -39,10 +39,6 @@ typedef union IDPropertyTemplate {
     /** #eIDPropertyType */
     char type;
   } array;
-  struct {
-    int matvec_size;
-    const float *example;
-  } matrix_or_vector;
 } IDPropertyTemplate;
 
 /* ----------- Property Array Type ---------- */

source/blender/makesdna/DNA_ID.h

@@ -127,9 +127,8 @@ typedef struct IDProperty {
   /** Size matches #MAX_IDPROP_NAME. */
   char name[64];
 
-  /* saved is used to indicate if this struct has been saved yet.
-   * seemed like a good idea as a '_pad' var was needed anyway :) */
-  int saved;
+  char _pad0[4];
+
   /** NOTE: alignment for 64 bits. */
   IDPropertyData data;