Remove a few unused global variables and declarations.

- Commit 3eb77eba5a, which moved the pending ops queue from md.c to
  sync.c, introduced a duplicate, unused 'pendingOpsCxt'
  variable. (I'm surprised none of the compilers or static analysis
  tools have complained about that.)

- Commit c2fe139c20 moved the 'synchronize_seqscans' variable and
  introduced an extern declaration in tableam.h, making the one in
  guc_tables.c unnecessary.

- Commit 6f0cf87872 removed the 'pgstat_temp_directory' GUC, but
  forgot to remove the corresponding global variable.

- Commit 1b4e729eaa removed the 'pg_krb_realm' GUC, and its global
  variable, but forgot the declaration in auth.h.

Spotted all these by reading the code.

src/backend/storage/sync/sync.c

@@ -37,8 +37,6 @@
 #include "utils/inval.h"
 #include "utils/memutils.h"
 
-static MemoryContext pendingOpsCxt; /* context for the pending ops state  */
-
 /*
  * In some contexts (currently, standalone backends and the checkpointer)
  * we keep track of pending fsync operations: we need to remember all relation