[flutter_markdown] Remove ignore: avoid_init_to_null since the package uses Dart 3 (#4852)

As the removed comment says, since the package uses Dart 3 we can remove the `ignore: avoid_init_to_null`. 

Fixes flutter/flutter#134058

Author: nilsreichardt

packages/flutter_markdown/lib/src/widget.dart

@@ -413,9 +413,6 @@ class MarkdownBody extends MarkdownWidget {
     required super.data,
     super.selectable,
     super.styleSheet,
-    // TODO(stuartmorgan): Remove this once 3.0 is no longer part of the
-    // legacy analysis matrix; it's a false positive there.
-    // ignore: avoid_init_to_null
     super.styleSheetTheme = null,
     super.syntaxHighlighter,
     super.onTapLink,
@@ -470,9 +467,6 @@ class Markdown extends MarkdownWidget {
     required super.data,
     super.selectable,
     super.styleSheet,
-    // TODO(stuartmorgan): Remove this once 3.0 is no longer part of the
-    // legacy analysis matrix; it's a false positive there.
-    // ignore: avoid_init_to_null
     super.styleSheetTheme = null,
     super.syntaxHighlighter,
     super.onTapLink,