Hi,
While trying to remove the Azure AD button from the UI I've tried several things and finally achieved it by disabling the Azure AD authentication.
I did the following:
- Accessed: https://make.preview.powerapps
- Click at the three dots in the Portal app
- Choose Settings > Authentication settings
- Disable Azure Active Directory identity provider
In fact, It did work, but now I want to change some texts in the Forgot Password page and I need to login with AD to achieve that (maybe there is another way?).
So I went through the same steps as before, but now to enabled the Azure Active Directory identity provider, and as you can imagine by now, it didn't work.
Here are some screen shots for reference:

