The code used to check for a `homeserver.yaml` file and generate
a configuration (+ key) only if such a configuration file didn't exist.

Certain rare cases (setting up with one server name and then
changing to another) lead to `homeserver.yaml` being there,
but a `matrix.DOMAIN.signing.key` file missing (because the domain
changed).
A new signing key file would never get generated, because `homeserver.yaml`'s
existence used to be (incorrectly) satisfactory for us.

From now on, we don't mix things up like that.
We don't care about `homeserver.yaml` anymore, but rather
about the actual signing key.

The rest of the configuration (`homeserver.yaml` and
`matrix.DOMAIN.log.config`) is rebuilt by us in any case, so whether
it exists or not is irrelevant and doesn't need checking.