Remove alter_algorithm

The reasons for removing alter-algorithm are:

• alter-algorithm was introduced as a replacement for the old_alter_table that was used
  to force the usage of the original alter table algorithm (copy) in the cases where the new
  alter algorithm did not work. The new option was added as a way to force the usage of
  a specific algorithm when it should instead have made it possible to disable algorithms
  that would not work for some reason.
• alter-algorithm introduced some cases where ALTER TABLE would not work without
  specifying the ALGORITHM=XXX option together with ALTER TABLE
• Having different values of alter-algorithm on master and slave could cause slave to
  stop unexpectedly.
• ALTER TABLE FORCE, as used by mariadb-upgrade, would not always work if
  alter-algorithm was set for the server.
• As part of the MDEV-33449 "improving repair of tables" it become clear that alter- algorithm made it harder to provide a better and more consistent ALTER TABLE FORCE and REPAIR TABLE and it would be better to remove it.

The task is to remove alter-algorithm but keep the variable as no-op to not affect old setups.