style: fix icon buttons are square in different sizes under the default shape

[qhanw]

🤔 这个变动的性质是？

• 新特性提交（功能补充）
• 日常 bug 修复
• 站点、文档改进
• 组件样式改进
• TypeScript 定义更新
• 重构
• 代码风格优化
• 测试用例
• 分支合并
• 其他改动（是关于什么的改动？）

🔗 相关 Issue

💡 需求背景和解决方案

1. 在使用纯图标按钮不设置形状时，按钮在默认尺寸下显示为正常正方形，而在size="large"与size="small"时则高宽一不相等，具体如下。现需要在其在默认形状下不同大小均显示为正方形，可根据icon only标记进行修复。
   

2.使用用式：<Button type="primary" icon="download" />。

📝 更新日志怎么写？

语言	更新描述
🇺🇸 英文
🇨🇳 中文	根据按钮在icon only情况下修复其在默认形状下展示方式

• 中文（可选）:

☑️ 请求合并前的自查清单

• 文档已补充或无须补充
• 代码演示已提供或无须提供
• TypeScript 定义已补充或无须补充
• Changelog 已提供或无须提供

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View rendered components/button/demo/button-group.md
View rendered components/button/demo/loading.md
View rendered components/button/demo/size.md
View rendered components/button/index.en-US.md
View rendered components/button/index.zh-CN.md

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[codecov]

Codecov Report

Merging #18458 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #18458   +/-   ##
=======================================
  Coverage   96.41%   96.41%           
=======================================
  Files         280      280           
  Lines        7522     7522           
  Branches     2089     2092    +3     
=======================================
  Hits         7252     7252           
  Misses        268      268           
  Partials        2        2

Impacted Files	Coverage Δ
components/button/button.tsx	93.96% <100%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #18458 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #18458   +/-   ##
=======================================
  Coverage   96.41%   96.41%           
=======================================
  Files         280      280           
  Lines        7522     7522           
  Branches     2089     2047   -42     
=======================================
  Hits         7252     7252           
  Misses        268      268           
  Partials        2        2

Impacted Files	Coverage Δ
components/button/button.tsx	93.96% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 271c7aa...af6b213. Read the comment docs.

[afc163]

不需要添加新 shape，icon only 的时候修正为正方形就行。

[qhanw]

不需要添加新 shape，icon only 的时候修正为正方形就行。

现己更正过来，新的 shape 己移除。