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/access/transam/xlog.c

@@ -59,7 +59,6 @@
 #include "access/xlog_internal.h"
 #include "access/xlogarchive.h"
 #include "access/xloginsert.h"
-#include "access/xlogprefetcher.h"
 #include "access/xlogreader.h"
 #include "access/xlogrecovery.h"
 #include "access/xlogutils.h"
@@ -79,7 +78,6 @@
 #include "postmaster/startup.h"
 #include "postmaster/walsummarizer.h"
 #include "postmaster/walwriter.h"
-#include "replication/logical.h"
 #include "replication/origin.h"
 #include "replication/slot.h"
 #include "replication/snapbuild.h"
@@ -90,12 +88,10 @@
 #include "storage/ipc.h"
 #include "storage/large_object.h"
 #include "storage/latch.h"
-#include "storage/pmsignal.h"
 #include "storage/predicate.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "storage/reinit.h"
-#include "storage/smgr.h"
 #include "storage/spin.h"
 #include "storage/sync.h"
 #include "utils/guc_hooks.h"
@@ -104,7 +100,6 @@
 #include "utils/memutils.h"
 #include "utils/ps_status.h"
 #include "utils/relmapper.h"
-#include "utils/pg_rusage.h"
 #include "utils/snapmgr.h"
 #include "utils/timeout.h"
 #include "utils/timestamp.h"