Fix unused variable warning in mlx5_fs_tree.c

With clang 15, the following -Werror warning is produced:

sys/dev/mlx5/mlx5_core/mlx5_fs_tree.c:1408:15: error: variable 'candidate_group_num' set but not used [-Werror,-Wunused-but-set-variable]
        unsigned int candidate_group_num = 0;
                     ^

The 'candidate_group_num' variable appears to have been a debugging aid
that has never been used for anything, so remove it.

MFC after: 3 days

(cherry picked from commit 6332ad8673fb108dcb8af0aaa42d75ea845748d7)