Remove redundant yet unstable test_spectral_lobpcg_mode

The lobpcg mode is already tested in the test_spectral_clustering test.

LinAlgError was raised when executing test_spectral_lobpcg_mode on some
systems depending on the version of the scipy and LAPACK libraries.
Merge pull request #3025 from ogrisel/remove-unstable-lobpcg-test

[MRG] Remove redundant yet unstable test_spectral_lobpcg_mode