Fix T96828: GPencil primitives handlers not working

Following the logic is not necessary to check the mouse button because the status has changed before.

source/blender/editors/gpencil/gpencil_primitive.c

@@ -1497,7 +1497,7 @@ static void gpencil_primitive_edit_event_handling(
       break;
     }
     case MOUSEMOVE: {
-      if ((event->val == KM_PRESS) && tgpi->sel_cp != SELECT_NONE) {
+      if (tgpi->sel_cp != SELECT_NONE) {
         if (tgpi->sel_cp == SELECT_START && tgpi->tot_stored_edges == 0) {
           copy_v2_v2(tgpi->start, tgpi->mval);
         }