Change confusing check discovered while working on HCL color wheel:

COLOR widgets never get created with UI_GRAD_V_ALT values. This is
intended for HSVCUBE color sliders only so reverting this to old value
before it was substituted with a define, which was -1.

There's one case where COLOR buttons get defined with a1 value different
than -1 which is in ui_def_but_rna. There the step value of the RNA
property is used. However, that meant that if step equaled to 9, which
is UI_GRAD_V_ALT value (unlikely) or if UI_GRAD_V_ALT was changed to 10
in future, or we added check for e.g. UI_GRAD_L_ALT which is 10 in HCL
branch, color widget would not get a color selection menu spawned.
HCL color wheel implementation.

This is actually a standard Hue - Saturation - Lightness model
(see for instance entry on wikipedia here: https://en.wikipedia.org/wiki/HSL_and_HSV)
but named S named C (Chroma) to disambiguate between HSV and HSL saturation, which are not the same

The advantage of having this color selection scheme is that artists can select
shades and tints of a color easily by using the lightness slider. Also colors
are arranged on (approximated) perceived lightness on the color wheel.

Beware, Old files opened with this preference saved will crash!