pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/110eb4aefbad683c8f512ee8a7168d1718353baa

Modified Files
--------------
contrib/pg_prewarm/autoprewarm.c | 4 ++--
src/backend/utils/activity/wait_event.c | 7 +++----
src/include/utils/wait_event.h | 6 ------
src/tools/pgindent/typedefs.list | 1 -
4 files changed, 5 insertions(+), 13 deletions(-)