style(showcase:select): remove unused variable

[tigercosmos]

remove unused variable in select: nz-demo-select-basic

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)
[ ] Refactoring (no functional changes, no api changes)
[X] 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?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[coveralls]

Coverage remained the same at 40.722% when pulling 1306d72 on tigercosmos:demo-dev into cd84e47 on NG-ZORRO:master.

[wilsoncook]

Pls normalize your commit message refer to https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit

[tigercosmos]

@wilsoncook thanks!