fix(module: tree): fix nz-tree-node keep dragging class with nzBeforeDrop

[simplejason]

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)
[ ] 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

[zorro-bot]

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[codecov]

Codecov Report

Merging #8015 (a0b93f2) into master (37391de) will decrease coverage by 0.03%.
The diff coverage is 75.00%.

❗ Current head a0b93f2 differs from pull request most recent head 0cd52f9. Consider uploading reports for the commit 0cd52f9 to get more accurate results

@@            Coverage Diff             @@
##           master    #8015      +/-   ##
==========================================
- Coverage   91.68%   91.66%   -0.03%     
==========================================
  Files         515      515              
  Lines       17634    17635       +1     
  Branches     2789     2789              
==========================================
- Hits        16168    16165       -3     
- Misses       1170     1172       +2     
- Partials      296      298       +2     

Impacted Files	Coverage Δ
components/tree/tree-node.component.ts	92.93% <0.00%> (-1.06%)	⬇️
components/dropdown/context-menu.service.ts	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes