[ast] Remove dead code identified by coverage reports

Change-Id: I8283ddf96dc178f12e52d9412db1858acfe46650
Reviewed-on: https://chromium-review.googlesource.com/c/1387966Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58447}

src/ast/ast.cc

@@ -70,15 +70,6 @@ IterationStatement* AstNode::AsIterationStatement() {
   }
 }
 
-BreakableStatement* AstNode::AsBreakableStatement() {
-  switch (node_type()) {
-    BREAKABLE_NODE_LIST(RETURN_NODE);
-    ITERATION_NODE_LIST(RETURN_NODE);
-    default:
-      return nullptr;
-  }
-}
-
 MaterializedLiteral* AstNode::AsMaterializedLiteral() {
   switch (node_type()) {
     LITERAL_NODE_LIST(RETURN_NODE);

src/ast/ast.h

@@ -161,7 +161,6 @@ class AstNode: public ZoneObject {
   FAILURE_NODE_LIST(DECLARE_NODE_FUNCTIONS)
 #undef DECLARE_NODE_FUNCTIONS
 
-  BreakableStatement* AsBreakableStatement();
   IterationStatement* AsIterationStatement();
   MaterializedLiteral* AsMaterializedLiteral();