Revert "Add MUI stub test fixture for sx migrator codemod"

This reverts commit eedbe7a

Author: greglittlefield-wf

test/resolved_file_context.dart

@@ -70,11 +70,6 @@ class SharedAnalysisContext {
   static final rmui = SharedAnalysisContext(
       p.join(findPackageRootFor(p.current), 'test/test_fixtures/rmui_project'));
 
-  /// A context root located at `test/test_fixtures/mui_stub_project`
-  /// that stubs out some MUI components using over_react.
-  static final muiStub = SharedAnalysisContext(p.join(
-      findPackageRootFor(p.current), 'test/test_fixtures/mui_stub_project'));
-
   /// The path to the package root in which test files will be created
   /// and resolved.
   final String _path;