Remove unused file.

This was added in 10540974, but has
never been used for anything as far as I can tell.  There seems to
be no reason to keep it.

src/include/storage/pos.h

-/*-------------------------------------------------------------------------
- *
- * pos.h
- *	  POSTGRES "position" definitions.
- *
- *
- * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
- * Portions Copyright (c) 1994, Regents of the University of California
- *
- * src/include/storage/pos.h
- *
- *-------------------------------------------------------------------------
- */
-#ifndef POS_H
-#define POS_H
-
-
-/*
- * a 'position' used to be <pagenumber, offset> in postgres.  this has
- * been changed to just <offset> as the notion of having multiple pages
- * within a block has been removed.
- *
- * the 'offset' abstraction is somewhat confusing.  it is NOT a byte
- * offset within the page; instead, it is an offset into the line
- * pointer array contained on every page that store (heap or index)
- * tuples.
- */
-typedef bits16 PositionIdData;
-typedef PositionIdData *PositionId;
-
-/* ----------------
- *		support macros
- * ----------------
- */
-
-/*
- * PositionIdIsValid
- *		True iff the position identifier is valid.
- */
-#define PositionIdIsValid(positionId) \
-	PointerIsValid(positionId)
-
-/*
- * PositionIdSetInvalid
- *		Make an invalid position.
- */
-#define PositionIdSetInvalid(positionId) \
-	*(positionId) = (bits16) 0
-
-/*
- * PositionIdSet
- *		Sets a position identifier to the specified value.
- */
-#define PositionIdSet(positionId, offsetNumber) \
-	*(positionId) = (offsetNumber)
-
-/*
- * PositionIdGetOffsetNumber
- *		Retrieve the offset number from a position identifier.
- */
-#define PositionIdGetOffsetNumber(positionId) \
-	((OffsetNumber) *(positionId))
-
-#endif   /* POS_H */