Remove webrender-wrench.

It's not that interesting.

Also, here's how long it takes to compile the entire thing:
```
- check  29.9s
- debug  43.0s
- opt    90s
```
And here's how long it takes to compile the final crate:
```
- check  0.9s
- debug  3.5s
- opt    9.5s
```
So the final crate is a small fraction of the overall time, which is a poor use
of CI resources.

Author: nnethercote