Commit dd299df8, which added suffix truncation to nbtree, simplified
the WAL record format used by page splits.  It became necessary to
explicitly WAL-log the new high key for the left half of a split in all
cases, which relieved the REDO routine from having to reconstruct a new
high key for the left page by copying the first item from the right
page.  Remove a comment that referred to the previous practice.