alternator: allow DescribeTimeToLive even without TTL enabled
alternator: allow DescribeTimeToLive even without TTL enabled

We still consider the TTL support in Alternator to be experimental, so we
don't want to allow a user to enable TTL on a table without turning on a
"--experimental-features" flag. However, there is no reason not to allow
the DescribeTimeToLive call when this experimental flag is off - this call
would simply reply with the truth - that the TTL feature is disabled for
the table!

This is important for client code (such as the Terraform module
described in issue #10660) which uses DescribeTimeToLive for
information, even when it never intends to actually enable TTL.

The patch is trivial - we simply remove the flag check in
DescribeTimeToLive, the code works just as before.

After this patch, the following test now works on Scylla without
experimental flags turned on:

    test/alternator/run test_ttl.py::test_describe_ttl_without_ttl

Refs #10660

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla