Reverts "Add tests for form_text_field.1.dart (flutter#150481)" (flutter#150696)

Reverts: flutter#150481
Initiated by: cbracken
Reason for reverting: Surprisingly, the following test seems to be consistently failing on Windows after the addition of this test:
```
flutter/packages/flutter/test/widgets/sliver_tree_test.dart: .toggleNodeWith, onNodeToggle
```
Original PR Author: ValentinVignal

Reviewed By: {TahaTesser, bleroux}

This change reverts the following previous change:
Contributes to flutter#130459

It adds a test for
- `examples/api/lib/material/text_form_field/text_form_field.1.dart`

Author: auto-submit[bot]

dev/bots/check_code_samples.dart

@@ -325,6 +325,7 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/flexible_space_bar/flexible_space_bar.0_test.dart',
   'examples/api/test/material/chip/deletable_chip_attributes.on_deleted.0_test.dart',
   'examples/api/test/material/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0_test.dart',
+  'examples/api/test/material/text_form_field/text_form_field.1_test.dart',
   'examples/api/test/material/scrollbar/scrollbar.1_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.0_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.1_test.dart',