Remove `start_offset_samples` and `stop_offset_samples` argument in `create_fixed_length_windows`

[gemeinl]

Instead or renaming the arguments as discussed in #260, we realized that these arguments do not make sense at all in the setting of create_fixed_length_windows. There, these arguments are equivalent to applying a preprocessing step that crops the signals in advance. We hence aim to remove the arguments at some point and deprecate them for now.