[Fonts] Fix outdated workaround in SkFontMgr

`remove_weak` would use `is_weak`, which used an outdated workaround for not having a method to check the binding of an FcPattern. Now that there exists an `FcPatternGetWithBinding`, we can use that instead.

Change-Id: I1ba74672a8f27630dce57d6396d5f675c89d3579
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1329557
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>