Remove duplicate using directive

This alias is already defined in src/ast/decoration.h and is probably
just a copy-paste error.

Change-Id: I9efc16881b551ccac287eef1f86ca7560e87131b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/48982
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/ast/struct_block_decoration.h

@@ -47,9 +47,6 @@ class StructBlockDecoration
   StructBlockDecoration* Clone(CloneContext* ctx) const override;
 };
 
-/// List of struct decorations
-using DecorationList = std::vector<Decoration*>;
-
 }  // namespace ast
 }  // namespace tint