Revert "Reenable unreachable-code warnings."

This reverts commit b292c30aa0.

Reason for revert: breaks Wuffs build

Original change's description:
> Reenable unreachable-code warnings.
>
> Change-Id: Ie56967a4b823388f0975384e88be23ff2fceecf7
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/504598
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: John Stiles <johnstiles@google.com>
> Auto-Submit: John Stiles <johnstiles@google.com>

Change-Id: I3844f535b3940a0408f0896ddf7ae9894328cba3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/504756
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

gn/skia/BUILD.gn

@@ -455,6 +455,7 @@ config("warnings") {
       "-Wno-switch-enum",
       "-Wno-thread-safety-negative",
       "-Wno-undef",
+      "-Wno-unreachable-code",
       "-Wno-unreachable-code-break",
       "-Wno-unreachable-code-return",
       "-Wno-unused-macros",