[MRG] remove useless warning in Random Projections

While RP is often used for dimensionality reduction, it's not necessarily the case. For instance the LSH pull request #3304 is using it internally to bucket the data.

Furthermore, if the user calls it with a fixed number of components (instead of using the "auto" mode that computes `n_components` based on the number of samples and the `eps` of the JL lemma), the users probably know what he/she is is doing so the warning is useless.

Therefore I think we should remove this warning.