fix(module:icon): resolve memory leak

[arturovt]

PR Checklist

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

[x] Bugfix

What is the current behavior?

I've noticed the NzIconService cannot be GC'd since it subscribes configUpdated$ which is a subject and keeps subscribers in its observers property until it's teardowned. The creates a memory leak for server-side rendered application since the NzIconService is created and destroyed there per each HTTP request.

What is the new behavior?

The NzIconService is GC'd successfully.

Does this PR introduce a breaking change?

[x] No

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #6839 (197efaa) into master (3267571) will decrease coverage by 0.05%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #6839      +/-   ##
==========================================
- Coverage   89.58%   89.53%   -0.06%     
==========================================
  Files         489      489              
  Lines       15790    15795       +5     
  Branches     2425     2426       +1     
==========================================
- Hits        14146    14142       -4     
- Misses       1003     1009       +6     
- Partials      641      644       +3     

Impacted Files	Coverage Δ
components/icon/icon.service.ts	92.42% <57.14%> (-4.30%)	⬇️
components/core/util/text-measure.ts	89.56% <0.00%> (-1.74%)	⬇️
components/tabs/tab-nav-bar.component.ts	82.33% <0.00%> (-1.42%)	⬇️

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[wzhudev]

LGTM. Thanks!