Revert b6002a796

This removes "Add Result Cache executor node".  It seems that something
weird is going on with the tracking of cache hits and misses as
highlighted by many buildfarm animals.  It's not yet clear what the
problem is as other parts of the plan indicate that the cache did work
correctly, it's just the hits and misses that were being reported as 0.

This is especially a bad time to have the buildfarm so broken, so
reverting before too many more animals go red.

Discussion: https://postgr.es/m/CAApHDvq_hydhfovm4=izgWs+C5HqEeRScjMbOgbpC-jRAeK3Yw@mail.gmail.com

src/include/nodes/pathnodes.h

@@ -1494,25 +1494,6 @@ typedef struct MaterialPath
 	Path	   *subpath;
 } MaterialPath;
 
-/*
- * ResultCachePath represents a ResultCache plan node, i.e., a cache that
- * caches tuples from parameterized paths to save the underlying node from
- * having to be rescanned for parameter values which are already cached.
- */
-typedef struct ResultCachePath
-{
-	Path		path;
-	Path	   *subpath;		/* outerpath to cache tuples from */
-	List	   *hash_operators; /* hash operators for each key */
-	List	   *param_exprs;	/* cache keys */
-	bool		singlerow;		/* true if the cache entry is to be marked as
-								 * complete after caching the first record. */
-	double		calls;			/* expected number of rescans */
-	uint32		est_entries;	/* The maximum number of entries that the
-								 * planner expects will fit in the cache, or 0
-								 * if unknown */
-} ResultCachePath;
-
 /*
  * UniquePath represents elimination of distinct rows from the output of
  * its subpath.
@@ -2110,9 +2091,6 @@ typedef struct RestrictInfo
 	Selectivity right_bucketsize;	/* avg bucketsize of right side */
 	Selectivity left_mcvfreq;	/* left side's most common val's freq */
 	Selectivity right_mcvfreq;	/* right side's most common val's freq */
-
-	/* hash equality operator used for result cache, else InvalidOid */
-	Oid			hasheqoperator;
 } RestrictInfo;
 
 /*