Remove redundant ModelElement.filePath implementations

Author: srawlins

lib/src/model/extension.dart

@@ -92,9 +92,6 @@ class Extension extends Container {
     ...typeParameters,
   ];
 
-  @override
-  String get filePath => '${library.dirName}/$fileName';
-
   @override
   String get sidebarPath => '${library.dirName}/$name-extension-sidebar.html';
 

lib/src/model/extension_type.dart

@@ -65,9 +65,6 @@ class ExtensionType extends InheritingContainer with Constructable {
     ...interfaceElements.expandInheritanceChain,
   ];
 
-  @override
-  String get filePath => '${library.dirName}/$fileName';
-
   @override
   String get fileName => '$name-extension-type.html';
 

lib/src/model/library.dart

@@ -184,12 +184,12 @@ class Library extends ModelElement
   ModelElement? get enclosingElement => null;
 
   @override
-  String get filePath => '${library.dirName}/$fileName';
+  String get filePath => '$dirName/$fileName';
 
   @override
   String get fileName => '$dirName-library.html';
 
-  String get sidebarPath => '${library.dirName}/$dirName-library-sidebar.html';
+  String get sidebarPath => '$dirName/$dirName-library-sidebar.html';
 
   /// The library template manually includes 'packages' in the left/above
   /// sidebar.