Avoid including tableam.h and xlogreader.h in nbtree.h

Avoid including tableam.h and xlogreader.h in nbtree.h

Doing that seems rather random and unnecessary.  This commit removes
those 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.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/202508051109.lzk3lcuzsaxo@alvherre.pgsql