Remove unnecessary checks for projections

This makes projection handling identical to _lcmv.py, simplifies the code, but
adds two unnecessary multiplications by an identity matrix in the (unlikely?)
case of no projections being present.

To justify why the checks are unnecessary:

* the 'projs' key is always expected to be in info
* if there are no projections, the info['projs'] is just an empty list
* if make_projector is passed an empty list, it just creates an identity array
Improve comment about regularization