Remove unused #include's from backend .c files

as determined by include-what-you-use (IWYU)

While IWYU also suggests to *add* a bunch of #include's (which is its
main purpose), this patch does not do that.  In some cases, a more
specific #include replaces another less specific one.

Some manual adjustments of the automatic result:

- IWYU currently doesn't know about includes that provide global
  variable declarations (like -Wmissing-variable-declarations), so
  those includes are being kept manually.

- All includes for port(ability) headers are being kept for now, to
  play it safe.

- No changes of catalog/pg_foo.h to catalog/pg_foo_d.h, to keep the
  patch from exploding in size.

Note that this patch touches just *.c files, so nothing declared in
header files changes in hidden ways.

As a small example, in src/backend/access/transam/rmgr.c, some IWYU
pragma annotations are added to handle a special case there.

Discussion: https://www.postgresql.org/message-id/flat/af837490-6b2f-46df-ba05-37ea6a6653fc%40eisentraut.org

src/backend/libpq/auth-scram.c

@@ -92,20 +92,15 @@
 #include <unistd.h>
 
 #include "access/xlog.h"
-#include "catalog/pg_authid.h"
 #include "catalog/pg_control.h"
 #include "common/base64.h"
 #include "common/hmac.h"
 #include "common/saslprep.h"
 #include "common/scram-common.h"
 #include "common/sha2.h"
-#include "libpq/auth.h"
 #include "libpq/crypt.h"
 #include "libpq/sasl.h"
 #include "libpq/scram.h"
-#include "miscadmin.h"
-#include "utils/builtins.h"
-#include "utils/timestamp.h"
 
 static void scram_get_mechanisms(Port *port, StringInfo buf);
 static void *scram_init(Port *port, const char *selected_mech,