Fix unused variable warning in mlx5_ib_devx.c

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

sys/dev/mlx5/mlx5_ib/mlx5_ib_devx.c:1926:6: error: variable 'num_alloc_xa_entries' set but not used [-Werror,-Wunused-but-set-variable]
        int num_alloc_xa_entries = 0;
            ^

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

MFC after: 3 days