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/utils/misc/guc_tables.c

@@ -97,7 +97,6 @@ extern char *default_tablespace;
 extern char *temp_tablespaces;
 extern bool ignore_checksum_failure;
 extern bool ignore_invalid_pages;
-extern bool synchronize_seqscans;
 
 #ifdef TRACE_SYNCSCAN
 extern bool trace_syncscan;
@@ -530,8 +529,6 @@ char	   *HbaFileName;
 char	   *IdentFileName;
 char	   *external_pid_file;
 
-char	   *pgstat_temp_directory;
-
 char	   *application_name;
 
 int			tcp_keepalives_idle;