l10n-zh: remove https://mdn.mozillademos.org references

[yin1999]

Related issues: #10400. The references to https://mdn.mozillademos.org (in the current context: path starts with \/files\/\d+) are broken now, we need to remove all of those.

regex pattern to search the reference: \/files\/\d+

### zh-CN
- [ ] files/zh-cn/learn/forms/styling_web_forms/index.md
- [ ] #20071
- [ ] #19648
- [ ] files/zh-cn/web/css/mask-border-slice/index.md
- [x] files/zh-cn/web/css/padding-bottom/index.md #19886
- [x] files/zh-cn/web/css/padding-top/index.md #19886
- [ ] #19626
- [ ] #19342
- [ ] #19186
- [ ] #19114
- [ ] #19089

### zh-TW
- [x] files/zh-tw/web/api/canvas_api/tutorial/applying_styles_and_colors/index.md #19120
- [x] files/zh-tw/web/api/canvas_api/tutorial/basic_animations/index.md #19120
- [x] files/zh-tw/web/api/setinterval/index.md #19120
- [x] files/zh-tw/web/css/_colon_target/index.md #19120
- [x] files/zh-tw/web/css/css_flexible_box_layout/basic_concepts_of_flexbox/index.md #19120
- [x] files/zh-tw/web/css/shorthand_properties/index.md #19120
- [x] files/zh-tw/web/security/mixed_content/index.md #19120