Remove carrier density noise from pygwinc

Based on the new calculation with Debye screening from arXiv:2003.05345, there appears to be no plausible scenario where this noise will affect the design of GW interferometers. Koji estimated the noise is below 1e-28/rtHz for Voyager, as did Florian Bruns (lead author of arXiv:2003.05345). It's also negligible for ET, as discussed in the paper.

Fixes #50 (closed)