pgsql: Remove a few unused global variables and declarations.

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.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/548d7260309008b146bd9eaa66f3c5be0a8d725a

Modified Files
--------------
src/backend/storage/sync/sync.c     | 2 --
src/backend/utils/misc/guc_tables.c | 3 ---
src/include/libpq/auth.h            | 1 -
3 files changed, 6 deletions(-)