Revert "Allow on-line enabling and disabling of data checksums"

This reverts the backend sides of commit 1fde38beaa.
I have, at least for now, left the pg_verify_checksums tool in place, as
this tool can be very valuable without the rest of the patch as well,
and since it's a read-only tool that only runs when the cluster is down
it should be a lot safer.

src/include/access/xlog_internal.h

@@ -25,7 +25,6 @@
 #include "lib/stringinfo.h"
 #include "pgtime.h"
 #include "storage/block.h"
-#include "storage/checksum.h"
 #include "storage/relfilenode.h"
 
 
@@ -241,12 +240,6 @@ typedef struct xl_restore_point
 	char		rp_name[MAXFNAMELEN];
 } xl_restore_point;
 
-/* Information logged when checksum level is changed */
-typedef struct xl_checksum_state
-{
-	ChecksumType new_checksumtype;
-}			xl_checksum_state;
-
 /* End of recovery mark, when we don't do an END_OF_RECOVERY checkpoint */
 typedef struct xl_end_of_recovery
 {