pgsql: Remove unused include file, per ITAGAKI Takahiro.

Log Message:
-----------
Remove unused include file, per ITAGAKI Takahiro. AFAICT this has been
dead code since Postgres95.

Removed Files:
-------------
pgsql/src/include/storage:
itempos.h
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/itempos.h)