Remove CORS configuration from subscription

[mthenw]

What did you implement:

Part of #438

How did you implement it:

It removes the remainings of CORS configuration in subscription model.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: YES

[codecov]

Codecov Report

Merging #458 into master will decrease coverage by 0.11%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
- Coverage   66.17%   66.06%   -0.12%     
==========================================
  Files          32       32              
  Lines        1892     1877      -15     
==========================================
- Hits         1252     1240      -12     
+ Misses        587      585       -2     
+ Partials       53       52       -1

Impacted Files	Coverage Δ
internal/cache/target.go	0% <0%> (ø)	⬆️
internal/cache/subscription_cache.go	86.76% <100%> (ø)	⬆️
libkv/subscription.go	88.6% <100%> (+0.58%)	⬆️
internal/pathtree/tree.go	97.22% <87.5%> (-0.13%)	⬇️

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