cmd/metadata: do not show size for node count

[maxmouchet]

It's a node count, not bytes :-)

The actual tree size (node count * node size) is already shown in the next line.

mmdbctl metadata proxy_residential.mmdb
...
- Node Count    920825782 (878.17 MB) # <--- Remove this size
- Tree Size     7366606256 (6.86 GB)
- Data Section Size 1774958 (1.69 MB)
...