4️⃣ [Flyouts] Clean up deprecated API parts

The Flyouts API has accumulated some deprecated API parts, and we should add these to the [EUI deprecation schedule](https://github.com/elastic/eui/issues/1469) to be removed (maybe in 3 months).

## 1. `hideTitle`
Flyouts require `title` for listing in history controls. Currently, the title is not visible in plain flyouts without history controls. However, the Discover Doc Viewer requires the title to be shown in the flyout menu. To achieve this, it is able to set `hideTitle: false` in the flyout menu props. This is to support a temporary design.

After Discover Doc Viewer implements the new design of featuring the title in the flyout header, we need clean up to remove ability to send `hideTitle: false` from the flyout props. This issue is to track that clean up.

----

| Before (with `hideTitle: false`) | After |
| :---: | :---: |
| | |

## 2. `customActions` and `EuiFlyoutMenuCustomAction` in `EuiFlyoutMenu`

Context is [here](https://github.com/elastic/eui/pull/9856#discussion_r3703059145)