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/pgstat.h

@@ -710,9 +710,7 @@ typedef enum BackendType
 	B_STARTUP,
 	B_WAL_RECEIVER,
 	B_WAL_SENDER,
-	B_WAL_WRITER,
-	B_CHECKSUMHELPER_LAUNCHER,
-	B_CHECKSUMHELPER_WORKER
+	B_WAL_WRITER
 } BackendType;