Remove unnecessary self from user model

What does this MR do?

Clear code in user model

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

self keyword nod needed. Code is more clean and understandable. https://github.com/bbatsov/ruby-style-guide#no-self-unless-required

• Changelog entry added
• Tests
  • n/a Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together