Remove double quotes from the second column of wait_event_names.txt

Remove double quotes from the second column of wait_event_names.txt

The double quotes used for the wait event names are not required, as
the values quoted are made of single words.  The files generated by
generate-wait_event_types.pl (pgstat_wait_event.c, wait_event_types.h
and wait_event_types.sgml) are exactly the same before and after this
commit, hence the wait event names and the enum elements have the same
names as before.

Discussion: https://postgr.es/m/ZK9S3jFEV1X797Ll@paquier.xyz