app: remove current implementation of GimpSpinScale soft bounds.
Mitch gets a better idea to deal with soft limits (i.e. min/max values different from the property min/max) applied to a spin scale created by gimp_prop_spin_scale_new(). So let's just remove the current implementation (using locale data on the GimpConfig object). The symmetry spin scales are back with crazy huge maximums, which makes quite a horrible GUI, but this is only temporary until Mitch commits his new implementation.
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