refactor: remove unneeded deleted_at in $allowedFields

[kenjis]

Description

• deleted_at is not needed for soft deletes.

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide

[kenjis]

@paulbalandan Do you know where does array{username: string, status: string, status_message: string, active: bool, last_active: string} come from?
See https://github.com/codeigniter4/shield/pull/963/files#diff-459f49e48ae4f20464cf08fd3256652536435610f4f7db89305fe7269e539ac9

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  Line   src/Models/UserModel.php                                                                                  
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  223    Offset 'email' does not exist on array{username: string, status: string, status_message: string, active:  
         bool, last_active: string}.                                                                               
  224    Cannot unset offset 'email' on array{username: string, status: string, status_message: string, active:    
         bool, last_active: string}.                                                                               
  225    Offset 'password_hash' does not exist on array{username: string, status: string, status_message: string,  
         active: bool, last_active: string}.                                                                       
  226    Cannot unset offset 'password_hash' on array{username: string, status: string, status_message: string,    
         active: bool, last_active: string}.                                                                       
 ------ ---------------------------------------------------------------------------------------------------------- 

[kenjis]

I created an issue: CodeIgniter/phpstan-codeigniter#19