Refactor: Simplify ID Property freeing

This also makes `IDP_CopyProperty` the "opposite"
of `IDP_FreeProperty`, which is what I'd expect.

Two refactoring steps:
* rename IDP_FreeProperty to IDP_FreePropertyContent
* new IDP_FreeProperty function that actually frees the property

Reviewers: brecht

Differential Revision: https://developer.blender.org/D4872

source/blender/python/intern/bpy_rna.c

@@ -1122,7 +1122,6 @@ static void pyrna_struct_dealloc(BPy_StructRNA *self)
 #ifdef PYRNA_FREE_SUPPORT
   if (self->freeptr && self->ptr.data) {
     IDP_FreeProperty(self->ptr.data);
-    MEM_freeN(self->ptr.data);
     self->ptr.data = NULL;
   }
 #endif /* PYRNA_FREE_SUPPORT */