[PL/HT] Rm unneeded docu. for density and viscosity.

Documentation/ProjectFile/prj/processes/process/HT/fluid/i_fluid.md

-Tag that encloses the specification of the fluid properties density and viscosity.
+Defines fluid properties.

Documentation/ProjectFile/prj/processes/process/HT/fluid/t_density.md

-Configuration for the constitutive laws for the fluid density from MaterialLib.

Documentation/ProjectFile/prj/processes/process/HT/fluid/t_viscosity.md

-A tag describing the viscosity law for the process at hand.

Documentation/ProjectFile/prj/processes/process/HT/t_specific_heat_capacity_fluid.md

-Specific heat capacity of the fluid.