[MRG] Remove redundant yet unstable test_spectral_lobpcg_mode

The lobpcg mode is already tested in the `test_spectral_clustering` test at [that line](https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/cluster/tests/test_spectral.py#L35).

`LinAlgError` is raised when executing `test_spectral_lobpcg_mode` on some
systems depending on the version of the scipy and LAPACK libraries.