Orphaned wait event

Hi,

Commit dee663f7 made WAIT_EVENT_SLRU_FLUSH_SYNC redundant, so here's a
patch to remove it.

In case it's useful again, here's how I noticed:

for X in ` grep WAIT_EVENT_ src/include/utils/wait_event.h |
           sed '/^#/d;s/,//;s/ = .*//' `
do
  if ! ( git grep $X |
         grep -v src/include/utils/wait_event.h |
         grep -v src/backend/utils/activity/wait_event.c |
         grep $X > /dev/null )
  then
    echo "$X is not used"
  fi
done