rust / build: Remove the `rust` tool

r? @pcwalton Sadly, there's a lack of resources for maintaining the `rust` tool,
and we decided in the 2013-10-08 Rust team meeting that it's better
to remove it altogether than to leave it in a broken state.

This deletion is without prejudice. If a person or people appear who
would like to maintain the tool, we will probably be happy to
resurrect it!

Closes #9775