refactor(module:rate): remove deprecated observers

[Nicoss54]

PR Checklist

Please check if your PR fulfills the following requirements:

• 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

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[X] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Currently we used observers.length to check if something subscribe to a subject. Unfortunatly this is deprecated in will be remove in the next version of rxjs (v8).

Observers will become internal.

Issue Number: N/A

What is the new behavior?

Use the new isObserved getter to check if something subscribe to a Subject

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

[zorro-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.60%. Comparing base (116346f) to head (179b4f8).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8454      +/-   ##
==========================================
- Coverage   91.62%   91.60%   -0.02%     
==========================================
  Files         530      530              
  Lines       18390    18390              
  Branches     2815     2815              
==========================================
- Hits        16849    16847       -2     
- Misses       1225     1226       +1     
- Partials      316      317       +1     

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[HyperLife1119]

This 'observed' property does not yet exist in rxjs6. Since angular/core still supports rxjs6, we can't do this yet.

[Nicoss54]

This 'observed' property does not yet exist in rxjs6. Since angular/core still supports rxjs6, we can't do this yet.

lets keep it in draft mode and blocked if you are agree ?

[HyperLife1119]

Totally agree.