remove final declaration in WetCoilCounterFlow

[mwetter]

This issue is to change

    redeclare final model HexElement =
      Buildings.Fluid.HeatExchangers.BaseClasses.HexElementLatent(simplify_mWat_flow=simplify_mWat_flow));

to

    redeclare model HexElement =
      Buildings.Fluid.HeatExchangers.BaseClasses.HexElementLatent(simplify_mWat_flow=simplify_mWat_flow));

in Fluid.WetCoilCounterFlow.

See also issue 2676 in https://github.com/modelica/ModelicaSpecification

This will be changed on the maint_7.0.x and the master branch.