Simplify spectral clustering solver logic

We need to simplify the logic for selecting a solver in ``spectral_clustering``.
See discussion here:
https://github.com/scikit-learn/scikit-learn/issues/10715#issuecomment-369236982
https://github.com/scikit-learn/scikit-learn/pull/14647#issuecomment-521294241
https://github.com/scikit-learn/scikit-learn/pull/10720#issuecomment-518878136