Remove unused SwitchStatement method.

The SwitchStatement has an IsDefault method which checks if the
condition is nullptr. The method is never called, and default
cases are tracked in the CaseStatement class, so it's questionable
what IsDefault even means here.

Change-Id: I96e97fbbf4823df3e92d2440fa6a9541a5ba337f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/106280
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/ast/switch_statement.h

@@ -38,9 +38,6 @@ class SwitchStatement final : public Castable<SwitchStatement, Statement> {
     SwitchStatement(SwitchStatement&&);
     ~SwitchStatement() override;
 
-    /// @returns true if this is a default statement
-    bool IsDefault() const { return condition == nullptr; }
-
     /// Clones this node and all transitive child nodes using the `CloneContext`
     /// `ctx`.
     /// @param ctx the clone context