Don't flush rewrite_rules on every admin request

[diedexx]

This PR fixes an issue where flush_rewrite_rules is called on every wp-admin pageload.

$wp_rewrite->extra_rules_top only includes rules that were registered during the current request. And because add_rewrite_rule is called after the logic that checks for the same rule on extra_rules_top, it will never exist and always call flush_rewrite_rules.

Instead, I suggest to do the check against $wp_rewrite->wp_rewrite_rules(), as that fetches the existing rules from the database.

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[codecov-commenter]

Codecov Report

Merging #544 (acd973f) into develop (f9fc1cb) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@              Coverage Diff              @@
##             develop     #544      +/-   ##
=============================================
- Coverage      23.26%   23.25%   -0.02%     
  Complexity       364      364              
=============================================
  Files             56       56              
  Lines           1947     1948       +1     
=============================================
  Hits             453      453              
- Misses          1494     1495       +1     

Flag	Coverage Δ
php	23.25% <0.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
wp-includes/class-wp.php	13.33% <0.00%> (-0.96%)	⬇️

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[moorscode]

@googlebot I signed it!

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[westonruter]

@moorscode The CLA bot needs you to sign as well now.

[moorscode]

@googlebot I fixed it.

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[diedexx]

@googlebot I consent.

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[westonruter]

Thank you!

And sorry for the hassle with the CLA.

[pooja-muchandikar]

QA Passed ✅

The changes are working as expected.