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

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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