Remove column for wait event names in wait_event_names.txt

Remove column for wait event names in wait_event_names.txt

This file is now made of two columns, removing the column listing the
user-visible strings used in the system views and the documentation:
- Enum definitions for each class without the prefix "WAIT_EVENT_", so
as this information can be grepped in the code and wait_event_names.txt
at the same time.
- Description in the documentation.

The wait event names are now generated from the enum objects in
CamelCase, with the underscores removed.  The data generated for wait
events is consistent with what was produced by 414f6c0fb79a.

This has the advantage to remove WAIT_EVENT_DOCONLY, which was a
placeholder for the wait event types Lock and LWLock as these two only
require the generation of the documentation.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/ZOxVHQwEC/9X/p/z@paquier.xyz