Remove deprecated preview policy flag

[shaun-nx]

Proposed changes

The -enable-preview-policies flag has been deprecated since the addition of the -enable-oidc flag here

This PR fully removes support for this flag.

Resolves: #4573

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[codecov]

Codecov Report

Merging #4595 (5c70c19) into main (bcc392a) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4595      +/-   ##
==========================================
+ Coverage   51.98%   51.99%   +0.01%     
==========================================
  Files          59       59              
  Lines       16967    16962       -5     
==========================================
  Hits         8820     8820              
+ Misses       7852     7847       -5     
  Partials      295      295              

Files	Coverage Δ
cmd/nginx-ingress/flags.go	30.88% <ø> (+0.73%)	⬆️

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