Remove infrastructure for fallthrough

This Cl removes the internal infrastructor and backend code
generation for the fallthrough statement.

Bug: tint:1644
Change-Id: I2a1de7d527865e5a7221074f4e0fb106599f4c57
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/109005
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

src/tint/sem/behavior.cc

@@ -24,8 +24,6 @@ std::ostream& operator<<(std::ostream& out, Behavior behavior) {
             return out << "Break";
         case Behavior::kContinue:
             return out << "Continue";
-        case Behavior::kFallthrough:
-            return out << "Fallthrough";
         case Behavior::kNext:
             return out << "Next";
     }

src/tint/sem/behavior.h

@@ -25,7 +25,6 @@ enum class Behavior {
     kReturn,
     kBreak,
     kContinue,
-    kFallthrough,
     kNext,
 };