hotfix: error raised for short series

[MMenchero]

Description

This hotfix removes an error that shouldn't be raised when dealing with short series. The original code included this:

if (any(input_samples > num_rows$initial_size)){
      stop(paste0("Your time series is too short. Please make sure that each of your series contains at least ", model_params$input_size+model_params$horizon, " observations."))
}

This part was added to prevent some unique ids from disappearing when using dplyr::slice_tail(). However, this is not necessary since the documentation of this function says that indices beyond the number of rows in the input are silently ignored. Furthermore, this was preventing some users from generating a forecast when the Python SDK had raised no issues for the same case.