fix: remove body font-size

[paranoidjk]

• rem 高清方案的用户，引入高清方案脚本来设置 html font size
• 非高清方案的用户，antd-mobile 设计规范应该对全局的 font-size 没要求

@peter0011

First of all, thank you for your contribution! :-)

Please makes sure that these checkboxes are checked before submitting your PR, thank you!

• Make sure that you follow antd's code convention.
• Run npm run lint and fix those errors before submitting in order to keep consistent code style.
• Rebase before creating a PR to keep commit history clear.
• Add some descriptions and refer relative issues for you PR.

Extra checklist:

if isBugFix :

• Make sure that you add at least one unit test for the bug which you had fixed.

elif isNewFeature :

• Update API docs for the component.
• Update/Add demo to demonstrate new feature.
• Update TypeScript definition for the component.
• Add unit tests for the feature.

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This change is 

[codecov]

Codecov Report

Merging #2106 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2106   +/-   ##
======================================
  Coverage    58.1%   58.1%           
======================================
  Files         253     253           
  Lines        4399    4399           
  Branches     1156    1156           
======================================
  Hits         2556    2556           
  Misses       1842    1842           
  Partials        1       1

Flag	Coverage Δ
#rn	60.42% <ø> (ø)	⬆️
#web	56.11% <ø> (ø)	⬆️

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