Remove large fill factor support from dynahash.c.

Since ancient times we have had support for a fill factor (maximum load
factor) to be set for a dynahash hash table, but:

1.  It was an integer, whereas for in-memory hash tables interesting
load factor targets are probably somewhere near the 0.75-1.0 range.

2.  It was implemented in a way that performed an expensive division
operation that regularly showed up in profiles.

3.  We are not aware of anyone ever having used a non-default value.

Therefore, remove support, effectively fixing it at 1.

Author: Jakub Wartak <Jakub.Wartak@tomtom.com>
Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/VI1PR0701MB696044FC35013A96FECC7AC8F62D0%40VI1PR0701MB6960.eurprd07.prod.outlook.com

src/include/utils/hsearch.h

@@ -68,7 +68,6 @@ typedef struct HASHCTL
 	long		ssize;			/* segment size */
 	long		dsize;			/* (initial) directory size */
 	long		max_dsize;		/* limit to dsize if dir size is limited */
-	long		ffactor;		/* fill factor */
 	Size		keysize;		/* hash key length in bytes */
 	Size		entrysize;		/* total user element size in bytes */
 	HashValueFunc hash;			/* hash function */
@@ -83,7 +82,6 @@ typedef struct HASHCTL
 #define HASH_PARTITION	0x0001	/* Hashtable is used w/partitioned locking */
 #define HASH_SEGMENT	0x0002	/* Set segment size */
 #define HASH_DIRSIZE	0x0004	/* Set directory size (initial and max) */
-#define HASH_FFACTOR	0x0008	/* Set fill factor */
 #define HASH_ELEM		0x0010	/* Set keysize and entrysize */
 #define HASH_BLOBS		0x0020	/* Select support functions for binary keys */
 #define HASH_FUNCTION	0x0040	/* Set user defined hash function */