intel/brw: Delete fs_reg_alloc::discard_interference_graph()

Unused since commit 50519598ff.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30319>

src/intel/compiler/brw_fs_reg_allocate.cpp

@@ -287,7 +287,6 @@ private:
    void setup_inst_interference(const fs_inst *inst);
 
    void build_interference_graph();
-   void discard_interference_graph();
 
    brw_reg build_lane_offsets(const fs_builder &bld,
                              uint32_t spill_offset, int ip);
@@ -561,14 +560,6 @@ fs_reg_alloc::build_interference_graph()
       setup_inst_interference(inst);
 }
 
-void
-fs_reg_alloc::discard_interference_graph()
-{
-   ralloc_free(g);
-   g = NULL;
-   have_spill_costs = false;
-}
-
 brw_reg
 fs_reg_alloc::build_single_offset(const fs_builder &bld, uint32_t spill_offset, int ip)
 {