[exercise-fluidsys] remove obsolete plot density function

exercises/exercise-fluidsystem/components/plotdensityfunction.py

-#!usr/bin/env python
-import numpy as np
-import matplotlib.pyplot as plt
-
-# function to calculate rho dependent on pressure
-rho_min = 1440;
-rho_max = 1480;
-k = 5e-7;
-
-def rho(p):
-    return rho_min + (rho_max - rho_min)/(1 + rho_min*np.exp(-1.0*k*(rho_max - rho_min)*p));
-
-# sample pressure in range (1e4, 1e7) and compute corresponding densities
-p = np.logspace(4, 7, 100)
-r = rho(p)
-
-# plot density vs. pressure
-plt.semilogx(p, r)
-plt.show()