Remove unused import for fix compile warning

Fix this warning:
```
servo/src/components/gfx/platform/macos/font_list.rs:6:4: 6:42 warning: unused import, #[warn(unused_imports)] on by default
servo/src/components/gfx/platform/macos/font_list.rs:6 use font_context::FontContextHandleMethods;
```