Avoid many uses of unwrap in font_cache_thread.rs
Replaced `result.send(...blah...).unwrap()` with `let _ = result.send(...blah...);` in `components/gfx/font_cache_thread.rs`
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#12188
- [X] These changes do not require tests because @jdm said if it compiles it's good enough
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e3eeb643f0
