[wasm] Fix warnings about unused variables. (#33451)

[wasm] Fix warnings about unused variables. (#33451)

/s/mono2/mono/metadata/sgen-mono.c:2883:16: warning: unused variable
      'last_major_gc_warned' [-Wunused-variable]
        static gint32 last_major_gc_warned = -1;
                      ^
/s/mono2/mono/metadata/sgen-mono.c:2884:16: warning: unused variable
      'num_degraded' [-Wunused-variable]
        static gint32 num_degraded = 0;
                      ^
/s/mono2/mono/metadata/sgen-mono.c:2886:9: warning: unused variable
      'major_gc_count' [-Wunused-variable]
        gint32 major_gc_count = mono_atomic_load_i32 (&mono_gc_stats.maj...
               ^

Co-authored-by: jaykrell <jaykrell@users.noreply.github.com>