Reverts "Native ios context menu (#143002)" (#148237)

Reverts: flutter/flutter#143002
Initiated by: cbracken
Reason for reverting: unresolved docs links. See failure here: https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20docs_test/16540/overview

```
dartdoc:stdout: Generating docs for package flutter...
dartdoc:stderr:   error: unresolved doc reference [TextInput.showSystemContextMenu]
dartdoc:stderr:     from widgets.MediaQueryData.supportsShowingSystemContextMenu: (file:///b/s/w/ir/x/w/flutter/packages/flutt
Original PR Author: justinmc

Reviewed By: {Renzo-Olivares, hellohuanlin}

This change reverts the following previous change:
In order to work around the fact that iOS 15 shows a notification on pressing Flutter's paste button (https://github.com/flutter/flutter/issues/103163), this PR allows showing the iOS system context menu in text fields.

<img width="385" alt="Screenshot 2024-02-06 at 11 52 25 AM" src="https://github.com/flutter/flutter/assets/389558/d82e18ee-b8a3-4082-9225-cf47fa7f3674">

It is currently opt-in, which a user would typically do like this (also in example system_context_menu.0.dart):

```dart
      contextMenuBuilder: (BuildContext context, EditableTextState editableTextState) {
        // If supported, show the system context menu.
        if (SystemContextMenu.isSupported(context)) {
          return SystemContextMenu.editableText(
            editableTextState: editableTextState,
          );
        }
        // Otherwise, show the flutter-rendered context menu for the current
        // platform.
        return AdaptiveTextSelectionToolbar.editableText(
          editableTextState: editableTextState,
        );
      },
```

Requires engine PR https://github.com/flutter/engine/pull/50095.

## API changes

### SystemContextMenu
A widget that shows the system context menu when built, and removes it when disposed. The main high-level way that most users would use this PR. Only works on later versions of iOS.

### SystemContextMenuController
Used under the hood to hide and show a system context menu. There can only be one visible at a time.

### MediaQuery.supportsShowingSystemContextMenu
Sent by the iOS embedder to tell the framework whether or not the platform supports showing the system context menu. That way the framework, or Flutter developers, can decide to show a different menu.

### `flutter/platform ContextMenu.showSystemContextMenu`
Sent by the framework to show the menu at a given `targetRect`, which is the current selection rect.

### `flutter/platform ContextMenu.hideSystemContextMenu`
Sent by the framework to hide the menu. Typically not needed, because the platform will hide the menu when the user taps outside of it and after the user presses a button, but it handles edge cases where the user programmatically rebuilds the context menu, for example.

### `flutter/platform System.onDismissSystemContextMenu`
Sent by the iOS embedder to indicate that the system context menu has been hidden by the system, such as when the user taps outside of the menu.  This is useful when there are multiple instances of SystemContextMenu, such as with multiple text fields.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/14d88ee0c8f499aad52f62a6573d4c37e20b7885