Drop unused dep on pkg:built_value_generator

Author: kevmoo

webdev/lib/src/version.dart

@@ -1,5 +1,5 @@
 name: webdev
-version: 2.0.2
+version: 2.0.3-dev
 author: Dart Team <[email protected]>
 homepage: https://github.com/dart-lang/webdev
 description: >-
@@ -36,7 +36,6 @@ dependencies:
 dev_dependencies:
   build: ^1.0.0
   build_runner: ^1.0.0
-  built_value_generator: ^6.4.0
   build_web_compilers: '>=1.0.0 <3.0.0'
   build_verify: ^1.0.0
   build_version: ^2.0.0