don't include tableam.h in nbtree.h

I noticed that nbtree.h includes tableam.h.  This seems rather random
and unnecessary.  This patch removes it and fixes fallout, which is
pretty minimal.  We do need to add a forward declaration of struct
TM_IndexDeleteOp (whose full definition appears in tableam.h) so that
_bt_delitems_delete_check()'s declaration can use it.

Also remove xlogreader.h, which is AFAICS 100% useless here.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/