[CFE] Remove unused compiler option 'declaredVariables'

Change-Id: I78cf7fafa64310e600e864b17b64c4aa3faa7aaa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/509981
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

pkg/front_end/lib/src/api_prototype/compiler_options.dart

@@ -99,10 +99,6 @@ class CompilerOptions {
   /// List of names that are allowed in dynamic calls in dynamic modules.
   List<String> dynamicCallsSelectorAllowList = [];
 
-  /// The declared variables for use by configurable imports and constant
-  /// evaluation.
-  Map<String, String>? declaredVariables;
-
   /// The [FileSystem] which should be used by the front end to access files.
   ///
   /// All file system access performed by the front end goes through this
@@ -285,7 +281,6 @@ class CompilerOptions {
         other.dynamicInterfaceSpecificationUri) {
       return false;
     }
-    if (!equalMaps(declaredVariables, other.declaredVariables)) return false;
     if (fileSystem != other.fileSystem) return false;
     if (compileSdk != other.compileSdk) return false;
     // chaseDependencies aren't used anywhere, so ignored here.