vips7compat.h: remove OrcProgram and OrcExecutor stubs

Remove the confusing `OrcProgram` and `OrcExecutor` stubs to avoid
conflicts with `orc.h`. Should be safe, the deprecated `VipsVector`
and `VipsExecutor` APIs was tightly coupled to liborc and became
no-op after the adoption of Highway.

While this is a potential ABI break, this API was likely not used
outside libvips. The same ABI break could also occur in versions
prior to 8.15 when a binary was built with `-Dorc=disabled` instead
of `-Dorc=enabled`.

libvips/include/vips/vips7compat.h

@@ -36,20 +36,9 @@
 
 #include <vips/mask.h>
 
-/* The old deprecated VipsVector/VipsExecutor API required orc.
- * Avoid a possible ABI/API break with the adoption of highway.
- */
 #ifdef HAVE_ORC
 #include <orc/orc.h>
-#else
-typedef struct _OrcProgram {
-	/* Opaque */
-} OrcProgram;
-
-typedef struct _OrcExecutor {
-	char data[808];
-} OrcExecutor;
-#endif
+#endif /* HAVE_ORC */
 
 #ifdef __cplusplus
 extern "C" {
@@ -1694,13 +1683,17 @@ typedef struct {
 
 	int d1;
 
+#ifdef HAVE_ORC
 	OrcProgram *program;
+#endif /*HAVE_ORC*/
 
 	gboolean compiled;
 } VipsVector;
 
 typedef struct {
+#ifdef HAVE_ORC
 	OrcExecutor executor;
+#endif /*HAVE_ORC*/
 
 	VipsVector *vector;
 } VipsExecutor;