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/backend/replication/basebackup.c

@@ -1384,7 +1384,7 @@ sendFile(const char *readfilename, const char *tarfilename, struct stat *statbuf
 
 	_tarWriteHeader(tarfilename, NULL, statbuf, false);
 
-	if (!noverify_checksums && DataChecksumsNeedVerify())
+	if (!noverify_checksums && DataChecksumsEnabled())
 	{
 		char	   *filename;
 

src/backend/replication/logical/decode.c

@@ -199,7 +199,6 @@ DecodeXLogOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 		case XLOG_FPW_CHANGE:
 		case XLOG_FPI_FOR_HINT:
 		case XLOG_FPI:
-		case XLOG_CHECKSUMS:
 			break;
 		default:
 			elog(ERROR, "unexpected RM_XLOG_ID record type: %u", info);