#5728 "Username already taken" validation does not get disappear

[Chamila Madusani]

Module:
Admin > users

Description:
"Username already taken" validation does not get disappear till user clicks on save button even if user enter non existing Username.

Assume that Chmila is an existing usrename.
1. go to Admin >users
2. click on add button and fill all the required fields
3. Add chamila as the Username. > validation displays
4. now add 1 at the end of the username (chamila1) > still validation displays.

Note: This defect should be fixed in both Add/ Edit User screens.