Revert "MDEV-16768: fix blob key length"

This reverts commit 87609324

RocksDB was making invalid assumption about Field_blob::make_sort_key,
and the commit 87609324 changed Field_blob::make_sort_key to match
RocksDB assumptions.

It also unintentionaly broke sys_vars.max_sort_length_func

sql/field.cc

@@ -8546,9 +8546,7 @@ void Field_blob::sort_string(uchar *to,uint length)
         Store length of blob last in blob to shorter blobs before longer blobs
       */
       length-= packlength;
-
-      uint key_length = MY_MIN(buf.length(), length);
-      store_bigendian(key_length, to + length, packlength);
+      store_bigendian(buf.length(), to + length, packlength);
     }
 
 #ifdef DBUG_ASSERT_EXISTS

storage/rocksdb/mysql-test/rocksdb_rpl/r/rpl_blob_key.result

-include/master-slave.inc
-[connection master]
-CREATE TABLE t1 (b BLOB, i INT, KEY(b(8))) ENGINE=RocksDB;
-INSERT INTO t1 VALUES (REPEAT('a',9),1);
-UPDATE t1 SET i = 2;
-connection slave;
-connection master;
-DROP TABLE t1;
-include/rpl_end.inc

storage/rocksdb/mysql-test/rocksdb_rpl/t/rpl_blob_key.test

---source include/have_rocksdb.inc
---source include/have_binlog_format_row.inc
---source include/master-slave.inc
-
-CREATE TABLE t1 (b BLOB, i INT, KEY(b(8))) ENGINE=RocksDB;
-INSERT INTO t1 VALUES (REPEAT('a',9),1);
-
-UPDATE t1 SET i = 2;
-
---sync_slave_with_master
-
-# Cleanup
---connection master
-DROP TABLE t1;
---source include/rpl_end.inc