Remove enum WaitEventExtension

This enum was used to determine the first ID to use when assigning a
custom wait event for extensions, which is always 1.  It was kept so
as it would be possible to add new in-core wait events in the category
"Extension".  There is no such thing currently, so let's remove this
enum until a case justifying it pops up.  This makes the code simpler
and easier to understand.

This has as effect to switch back autoprewarm.c to use PG_WAIT_EXTENSION
rather than WAIT_EVENT_EXTENSION, on par with v16 and older stable
branches.

Thinko in c9af05465307.

Reported-by: Peter Eisentraut
Discussion: https://postgr.es/m/195c6c45-abce-4331-be6a-e87724e1d060@eisentraut.org

src/include/utils/wait_event.h

@@ -53,12 +53,6 @@ extern PGDLLIMPORT uint32 *my_wait_event_info;
  *
  * The ID retrieved can be used with pgstat_report_wait_start() or equivalent.
  */
-typedef enum
-{
-	WAIT_EVENT_EXTENSION = PG_WAIT_EXTENSION,
-	WAIT_EVENT_EXTENSION_FIRST_USER_DEFINED,
-} WaitEventExtension;
-
 extern void WaitEventExtensionShmemInit(void);
 extern Size WaitEventExtensionShmemSize(void);