Cleanup: Move attribute domain count out of enum

The number of attribute domains is not an attribute domain.
This way it doesn't have to be handled in switch statements.

Differential Revision: https://developer.blender.org/D15065

source/blender/blenkernel/BKE_attribute.h

@@ -30,9 +30,8 @@ typedef enum AttributeDomain {
   ATTR_DOMAIN_CORNER = 3,   /* Mesh Corner */
   ATTR_DOMAIN_CURVE = 4,    /* A single curve in a larger curve data-block */
   ATTR_DOMAIN_INSTANCE = 5, /* Instance */
-
-  ATTR_DOMAIN_NUM
 } AttributeDomain;
+#define ATTR_DOMAIN_NUM 6
 
 typedef enum AttributeDomainMask {
   ATTR_DOMAIN_MASK_POINT = (1 << 0),