dot: remove unneeded visited check
dot: remove unneeded visited check

The children pointer is unique to the node, so it is not possible to
have visited it before. The node guard is present at the beginning of
print_all_decls(), which is the only function that can call
print_decls().

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl