[ast/cleanup] Remove unused FunctionLiteral::SourceSize method

Change-Id: I091a1f4a1f2292b37a56520d0a5c46ac5781b459
Reviewed-on: https://chromium-review.googlesource.com/575515Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46725}

src/ast/ast.h

@@ -2404,7 +2404,6 @@ class FunctionLiteral final : public Expression {
   int function_token_position() const { return function_token_position_; }
   int start_position() const;
   int end_position() const;
-  int SourceSize() const { return end_position() - start_position(); }
   bool is_declaration() const { return function_type() == kDeclaration; }
   bool is_named_expression() const {
     return function_type() == kNamedExpression;