docs: Remove multiline and undo-redo test cases (#151)

* docs: Remove automated multi-line component test case

This test case was automated in:

- WordPress/gutenberg#49352
- WordPress/gutenberg#53075

The remaining known issues are captures in GitHub Issues.

* docs: Remove broken undo redo test link

This test was removed in 316b1b0.

test-cases/gutenberg/writing-flow/readme.md

@@ -2,12 +2,6 @@
 
 #### General
 - [ ] [TC001](https://github.com/wordpress-mobile/test-cases/blob/trunk/test-cases/gutenberg/writing-flow/copy-paste.md#tc001) - Paste formatted text copied from website
-- [ ] [TC001](https://github.com/wordpress-mobile/test-cases/blob/trunk/test-cases/gutenberg/writing-flow/multiline-components.md#tc001) - Multiline components
-  - [ ] Quote block
-  - [ ] Verse block
-  - [ ] Preformatted block
-  - [ ] Code block (DEV only)
-  - [ ] Pullquote block
 #### Rich Text Format
 - [ ] [TC007](https://github.com/wordpress-mobile/test-cases/blob/trunk/test-cases/gutenberg/writing-flow/rich-text-formatting.md#tc007) - Test format detection under the cursor
 - [ ] [TC009](https://github.com/wordpress-mobile/test-cases/blob/trunk/test-cases/gutenberg/writing-flow/rich-text-formatting.md#tc009) - Test autocorrection doesn't apply formatting to Heading
@@ -28,5 +22,3 @@
 - [ ] [TC005](https://github.com/wordpress-mobile/test-cases/blob/trunk/test-cases/gutenberg/writing-flow/splitting-merging.md#tc005) - Merge multiple blocks
   - [ ] Paragraph
   - [ ] Heading
-#### Undo / Redo - Test Cases
-- [ ] [TC002](https://github.com/wordpress-mobile/test-cases/blob/trunk/test-cases/gutenberg/writing-flow/undo-redo.md#tc002) - Undo/redo text