Remove `render_flash_notice` helper

[tvdeyen]

What is this pull request for?

Since we only used this internally we can simply remove this, instead of deprecating it.

If you used this helper anyway, please

render Alchemy::Admin::Message.new(message, type: flash_type, dismissable: true)

instead.

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.86%. Comparing base (f123201) to head (640ddc0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2857 +/- ## ========================================== - Coverage 95.86% 95.86% -0.01% ========================================== Files 229 229 Lines 6211 6209 -2 ========================================== - Hits 5954 5952 -2 Misses 257 257 ```

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