Fix Sculpt Vertex Colors sample color changing brush alpha

It is not practical to change the alpha of the paint color with the
color picker as with the current brush design alpha is the main strength
control for the brush.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8208

source/blender/editors/sculpt_paint/sculpt.c

@@ -8184,7 +8184,6 @@ static int sculpt_sample_color_invoke(bContext *C,
   copy_v3_v3(color_srgb, active_vertex_color);
   IMB_colormanagement_scene_linear_to_srgb_v3(color_srgb);
   BKE_brush_color_set(scene, brush, color_srgb);
-  BKE_brush_alpha_set(scene, brush, active_vertex_color[3]);
 
   WM_event_add_notifier(C, NC_BRUSH | NA_EDITED, brush);