RunModel_Lag: remove experimental code introduced in v1.7.8

I accidentally copy the code of an experimental version of RunModel_Lag from https://forge.inrae.fr/airgriwrm/airgriwrm/-/blob/1f2a31b9249eac9f9d15a13d902e22f83d086774/R/RunModel_Routing.R in airGR for the resolution of #191 (closed).

The version located at https://forge.inrae.fr/airgriwrm/airgriwrm/-/blob/dev/R/RunModel_Routing.R?ref_type=heads should be used.

There is no impact on the calculation, it's only for cleaning the code.