refactor: convert all LeetCode solutions to Go-only

Changes:
- Removed all Java code implementations
- Kept only Go language solutions
- Renamed "## Go 解法" to "## 解法"
- Removed "### Go 代码要点" sections
- Cleaned up duplicate headers and empty sections
- Streamlined documentation for better readability

Updated files (9):
- 三数之和.md
- 两数相加.md
- 无重复字符的最长子串.md
- 最长回文子串.md
- 括号生成.md
- 子集.md
- 单词搜索.md
- 电话号码的字母组合.md
- 柱状图中最大的矩形.md

All 22 LeetCode Hot 100 Medium problems now use Go exclusively.
Code is cleaner, more focused, and easier to follow.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

16-LeetCode Hot 100/电话号码的字母组合.md

@@ -151,7 +151,6 @@ func main() {
 }
 ```
 
-
 ```go
 func letterCombinationsIterative(digits string) []string {
 	if digits == "" {
@@ -191,7 +190,6 @@ func letterCombinationsIterative(digits string) []string {
 }
 ```
 
-
 - **时间复杂度：** O(3^m × 4^n)
   - 其中 m 是对应 3 个字母的数字个数（2, 3, 4, 5, 6, 8）
   - n 是对应 4 个字母的数字个数（7, 9）