MDEV-21870 Deprecate and ignore innodb_scrub_log and innodb_scrub_log_speed

The configuration parameter innodb_scrub_log never really worked, as
reported in MDEV-13019 and MDEV-18370.

Because MDEV-14425 is changing the redo log format, the innodb_scrub_log
feature would have to be adjusted for it. Due to the known problems,
it is easier to remove the feature for now, and to ignore and deprecate
the parameters.

If old log contents should be kept secret, then enabling innodb_encrypt_log
or setting a smaller innodb_log_file_size could help.

mysql-test/suite/encryption/r/innodb-scrub-log.result

@@ -1,3 +0,0 @@
-create table t1(a int not null primary key auto_increment,
-b varchar(200), c char(100), d varchar(150)) engine=innodb;
-DROP TABLE t1;