refactor(module:breadcrumb): remove redundant dom nodes

[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?

The nz-breadcrumb-separator has extra dom nodes.

Issue Number: N/A

What is the new behavior?

Remove redundant dom nodes in nz-breadcrumb-separator.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

This may break user custom styles, which may be rare:

nz-breadcrumb-separator .ant-breadcrumb-separator { }
nz-breadcrumb-separator > .ant-breadcrumb-separator { }
nz-breadcrumb-separator span { }
nz-breadcrumb-separator > span { }

Other information

This commit also fixes the documentation and simplifies some code.

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #8017 (b0577f0) into master (2d0b3f7) will not change coverage.
The diff coverage is n/a.

❗ Current head b0577f0 differs from pull request most recent head 6859d88. Consider uploading reports for the commit 6859d88 to get more accurate results

@@           Coverage Diff           @@
##           master    #8017   +/-   ##
=======================================
  Coverage   91.64%   91.64%           
=======================================
  Files         515      515           
  Lines       17637    17637           
  Branches     2789     2789           
=======================================
  Hits        16164    16164           
  Misses       1175     1175           
  Partials      298      298           

Impacted Files	Coverage Δ
components/breadcrumb/breadcrumb-item.component.ts	100.00% <ø> (ø)
...nents/breadcrumb/breadcrumb-separator.component.ts	100.00% <ø> (ø)

[HyperLife1119]

The PR needs a reviewer. @simplejason