Remove a few unused struct members.

Oversights in commits ab9e0e718a, f3049a603a, and 247ce06b88.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/aScUuBSawPWogUxs%40ip-10-97-1-34.eu-west-3.compute.internal

src/backend/libpq/auth-scram.c

@@ -134,8 +134,6 @@ typedef struct
 {
 	scram_state_enum state;
 
-	const char *username;		/* username from startup packet */
-
 	Port	   *port;
 	bool		channel_binding_in_use;
 

src/backend/storage/aio/method_worker.c

@@ -55,7 +55,6 @@
 typedef struct PgAioWorkerSubmissionQueue
 {
 	uint32		size;
-	uint32		mask;
 	uint32		head;
 	uint32		tail;
 	int			sqes[FLEXIBLE_ARRAY_MEMBER];

src/backend/utils/sort/sharedtuplestore.c

@@ -88,7 +88,6 @@ struct SharedTuplestoreAccessor
 	/* State for writing. */
 	SharedTuplestoreChunk *write_chunk; /* Buffer for writing. */
 	BufFile    *write_file;		/* The current file to write to. */
-	BlockNumber write_page;		/* The next page to write to. */
 	char	   *write_pointer;	/* Current write pointer within chunk. */
 	char	   *write_end;		/* One past the end of the current chunk. */
 };