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.
Rename an unused parameter to silence a warning.
auto merge of #2375 : zwarich/servo/progressevent-rename, r=metajack
Use == instead of match for tests against a single enum value

The performance of using == should now equal that of match, so many
identity methods can be simplified to a single line.

Fixes #1596.
auto merge of #2372 : mrobinson/servo/issue-1596, r=metajack

The performance of using == should now equal that of match, so many
identity methods can be simplified to a single line.

Fixes #1596.