refactor(event): remove legacy IOCP backend

[ithewei]

Summary

• remove the unmaintained native EVENT_IOCP backend and overlapio implementation
• remove IOCP-only state and conditional dispatch from the core event loop
• document the maintained Windows backends: wepoll by default and WSAPoll when WITH_WEPOLL=OFF

The native IOCP path stopped being the Windows default in 2020 and had no supported build option. Windows continues to use the existing readiness-based NIO path through wepoll or WSAPoll.

Verification

• cmake --build /private/tmp/libhv-remove-iocp-build-20260926 --parallel
• cmake --build /private/tmp/libhv-remove-iocp-tests-20260926 --target unittest --parallel
• make unittest
• bash scripts/unittest.sh
• MinGW cross-build with WITH_WEPOLL=ON
• MinGW cross-build with WITH_WEPOLL=OFF
• repository scan confirms no remaining EVENT_IOCP, overlapio, or event/iocp.c references outside the local implementation plan

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