Cassandra has deprecated their Thrift interface years ago (a few months
after Cassandra SE was released). The new way to access Cassandra is to
use the CQL Connector, which does not use Thrift.

There have been talks to make a CassandraSE V2 but no usable code have
been produced. The motivation to keep CassandraSE in the MariaDB source
code is to have it as an example for such future attempts.

It can stay in the source code but it should be removed from packaging
to avoid Debian packaging maintenance work around trying to compile it
and have tests pass on new platform.

If somebody wants Cassandra, they can use the 10.4 branch.