Remove an unused field from CodegenRust.py.

The defaultVisibility field was cloned from the C++ implementation,
where it tracks the difference between struct and class visibility.
Since no similar concept exists in Rust, it should be removed.
auto merge of #2373 : zwarich/servo/remove-default-visibility, r=Ms2ger

The defaultVisibility field was cloned from the C++ implementation,
where it tracks the difference between struct and class visibility.
Since no similar concept exists in Rust, it should be removed.