Remove enum WaitEventExtension

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