Fix linting with updated spectral version

Spectral removed auto detection for openAPI schemas. Because of this, the linting fails when no .spectra.yaml is provided.

See: https://github.com/stoplightio/spectral/issues/1796

In the Autobahn repo there is a .spectral.yaml file to account for that.

However, if we do not expect to use custom linting rulesets, we can simply get rid of this extra file by executing this before the linting process:

`echo "extends: spectral:oas" > .spectral.yaml`