iris: Don't assert a NULL aux BO during aux config

The assert was introduced in a function that allocated an auxiliary
surface BO, iris_resource_alloc_aux. After refactors, the function it's
in now, iris_resource_configure_aux, no longer does this allocation.
Drop the assert because its purpose is unclear and it's no longer
relevant for CCS on XeHP.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13555>

src/gallium/drivers/iris/iris_resource.c

@@ -804,8 +804,6 @@ iris_resource_configure_aux(struct iris_screen *screen,
       res->aux.sampler_usages = res->aux.possible_usages;
 
    enum isl_aux_state initial_state;
-   assert(!res->aux.bo);
-
    switch (res->aux.usage) {
    case ISL_AUX_USAGE_NONE:
       /* Having no aux buffer is only okay if there's no modifier with aux. */