Sculpt: Fix #104475, data transfer not respecting vgroup for float colors

This might've been a merge error, the result of color mixing
was being overwritten by a simple copy of source to destination
inside of layerCopyValue_propcol.

source/blender/blenkernel/intern/customdata.cc

@@ -1337,8 +1337,6 @@ static void layerCopyValue_propcol(const void *source,
       memcpy(tmp_col, m1->color, sizeof(tmp_col));
     }
     blend_color_interpolate_float(m2->color, m2->color, tmp_col, mixfactor);
-
-    copy_v4_v4(m2->color, m1->color);
   }
 }