I also remove (without replacement) 2 usages of gimp_get_data() without
associated gimp_set_data(). According to the comment, it looks like the
associated data was likely set by the GAP plug-in instead. Let's drop this.

Finally I see a lot of arguments, several of them being float arrays, so I
wondered if they were related to the curve and point arrays we are storing as
aux arg now. Apparently these arguments are not even used in the plug-in, right
now, which is explained in commit e318651c. Maybe if these were actually
used/set and if we implement float array (de)serialization, would this aux
argument become useless?

Lastly, I deleted some code paths which were never used. For instance, stored
bval.total_steps was never set to anything else than 0 anywhere (so I removed
both the variable and the other code-path, which actually ended up being the
GAP-related data get). Additionally in p_bender_calculate_iter_curve(), there
...