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/replication/logical/decode.c

@@ -26,22 +26,19 @@
  */
 #include "postgres.h"
 
-#include "access/heapam.h"
 #include "access/heapam_xlog.h"
 #include "access/transam.h"
 #include "access/xact.h"
 #include "access/xlog_internal.h"
 #include "access/xlogreader.h"
 #include "access/xlogrecord.h"
-#include "access/xlogutils.h"
 #include "catalog/pg_control.h"
 #include "replication/decode.h"
 #include "replication/logical.h"
 #include "replication/message.h"
-#include "replication/origin.h"
 #include "replication/reorderbuffer.h"
 #include "replication/snapbuild.h"
-#include "storage/standby.h"
+#include "storage/standbydefs.h"
 
 /* individual record(group)'s handlers */
 static void DecodeInsert(LogicalDecodingContext *ctx, XLogRecordBuffer *buf);