Remove sdl_ttf from sdl_mouse test

sdl_ttf isn't being used at all.

```
➜  tests git:(incoming) ✗ python runner.py browser.test_sdl_mouse
[Browser harness server on process 80314]

Running the browser tests. Make sure the browser allows popups from localhost.

test_sdl_mouse (test_browser.browser) ... ok
[Browser harness server terminated]

----------------------------------------------------------------------
Ran 1 test in 5.055s

OK
```