…emoving the EuiOverlayMask

Fixes [this issue](elastic#92798), introduced when [the EUI modal implementation changed](elastic/eui#4480), such that it's no longer necessary to wrap modals in an `EuiOverlayMask`. The mask is now built-in to `EuiModal`.

The change above became effective throughout Kibana when it was upgraded to use a newer version of EUI via [this commit on Feb 16](elastic@8126488#diff-7ae45ad102eab3b6d7e7896acd08c427a9b25b346470d7bc6507b6481575d519).

This PR resolves the issue by removing the `EuiOverlayMask` around the `Customize Event Renderers modal`, shown in the `After` screenshot below:

### Before

![before](https://user-images.githubusercontent.com/59917825/109154007-b2e23880-7793-11eb-83bb-4774df77c5d6.png)

### After

![after](https://user-images.githubusercontent.com/4459398/109561954-0c4fad80-7a9b-11eb-9283-51d50ec8ea26.png)

### Desk testing

Desk-tested on a 16" 2019 MBP, and on the desktop with the following browser versions:

- Chrome `88.0.4324.192`
- Firefox `86.0`
- Safari `14.0.3`

…rlayMask