remove references to unsupported openssh [skip ci]

[baentsch]

This removes no longer warranted references to oqs-openssh.

• [no] Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• [no] Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)

[baentsch]

Created this PR to "balance out" open-quantum-safe/openssh#157 (for the avoidance of doubt: I don't want to change the status of that sub project in the absence of a maintainer, but just cannot watch on users repeatedly having the same problems with "OQS stuff": We should reduce the time people need to invest to use PQC code, not waste it).

[SWilson4]

LGTM, with one minor addition suggested.

[dstebila]

Thanks Michael!