perf(module:date-picker): remove 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
[x] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #7767 (dd767b2) into master (e82fda7) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7767      +/-   ##
==========================================
- Coverage   92.05%   92.03%   -0.02%     
==========================================
  Files         502      502              
  Lines       16783    16783              
  Branches     2645     2645              
==========================================
- Hits        15449    15447       -2     
- Misses       1063     1067       +4     
+ Partials      271      269       -2     

Impacted Files	Coverage Δ
components/date-picker/date-picker.component.ts	94.42% <100.00%> (ø)
components/table/src/table/tbody.component.ts	88.88% <0.00%> (-11.12%)	⬇️
components/table/src/table/tr-measure.component.ts	86.95% <0.00%> (-4.35%)	⬇️
components/table/src/table-style.service.ts	96.55% <0.00%> (+1.72%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.