[rustdoc] Copy only item path to clipboard rather than full `use` statement.
The (somewhat) recent addition of the "copy item import to clipboard" button is extremely nice.
However, i tend to write my code with fully qualified paths wherever feasible and only resort to `use` statements as a refactoring pass. This makes the "copy to clipboard" workflow awkward to use, as i would be copy-pasting that as, say
```rust
impl use std::ops::Add; for MyType {
```
and then go back and remove the `use ` and `;`.
This PR removes the `use ;` decorations, making it much nicer to use for fully-qualified items. I argue, however, that this does not noticeably degrade experience for those who prefer to import items, since the hard part about those is getting the path right, and writing the `use ;` decoration can be done by hand with little effort.
