Remove the SECURITY_ROW_LEVEL_DISABLED security context bit.

This commit's parent made superfluous the bit's sole usage.  Referential
integrity checks have long run as the subject table's owner, and that
now implies RLS bypass.  Safe use of the bit was tricky, requiring
strict control over the SQL expressions evaluating therein.  Back-patch
to 9.5, where the bit was introduced.

Based on a patch by Stephen Frost.

src/include/utils/plancache.h

@@ -111,7 +111,6 @@ typedef struct CachedPlanSource
 	int			num_custom_plans;		/* number of plans included in total */
 	bool		hasRowSecurity; /* planned with row security? */
 	bool		row_security_env;		/* row security setting when planned */
-	bool		rowSecurityDisabled;	/* is row security disabled? */
 } CachedPlanSource;
 
 /*