perf(module:form): remove a unused variable

[HyperLife1119]

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?

Issue Number: N/A

What is the new behavior?

None.

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #7766 (f6ce3b2) into master (3a638af) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7766      +/-   ##
==========================================
- Coverage   92.02%   92.02%   -0.01%     
==========================================
  Files         502      502              
  Lines       16762    16783      +21     
  Branches     2638     2645       +7     
==========================================
+ Hits        15426    15445      +19     
+ Misses       1069     1068       -1     
- Partials      267      270       +3     

Impacted Files	Coverage Δ
components/form/form-control.component.ts	98.27% <100.00%> (ø)
components/tree-select/tree-select.component.ts	91.25% <0.00%> (-0.94%)	⬇️
components/tabs/tab-nav-bar.component.ts	87.23% <0.00%> (-0.71%)	⬇️
components/popconfirm/popconfirm.ts	97.80% <0.00%> (ø)
components/date-picker/date-picker.component.ts	94.42% <0.00%> (ø)
components/icon/icon.directive.ts	97.18% <0.00%> (+0.04%)	⬆️
components/tree/tree.component.ts	94.97% <0.00%> (+0.05%)	⬆️
components/alert/alert.component.ts	95.31% <0.00%> (+0.07%)	⬆️
components/image/image-preview.component.ts	94.30% <0.00%> (+0.11%)	⬆️
components/select/select.component.ts	91.46% <0.00%> (+0.16%)	⬆️
... and 1 more

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