Delete user modal for new author should filter out the user to be deleted from possibilities

The user picker displayed to chose a new author in case of user deletion, should never the user about to be deleted as new author.

Reproduction steps:

• Create an user, login and create pages with that user
• Login with Admin, go in Administration > User
• Disable the user and click on the cross to delete the user
• A modal is displayed to change the author of the page created by the user
• Type the name of the user to be deleted in the picker for new author

Expected result:

• The user about to be deleted should not be displayed

Obtained result:

• The user about to be deleted is displayed