Removed unused use UsernameNotFoundException
minor #4391 Removed unused use UsernameNotFoundException (boekkooi)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #4391).

Discussion
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Removed unused use UsernameNotFoundException

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4, 2.5, master
| Fixed tickets | -

Commits
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4e6256b Removed unused use UsernameNotFoundException