Remove auto-installation of mecab in setup.py and bump version to 6.0.5

[hyunwoongko]

Summary

This PR removes the automatic installation of mecab and related Python packages from setup.py and bumps the version from 6.0.4 to 6.0.5.

Motivation

• Fixes the issue where pip install kss or pip install . prompts for a password on macOS and Ubuntu, both inside and outside of virtual environments.
• Prevents privilege escalation and respects Python packaging best practices.
• Prepares for a new release (6.0.5) on PyPI.

Details

• The PreInstall class in setup.py no longer attempts to install mecab or its Python bindings automatically.
• Users are now expected to install mecab and its Python bindings manually if needed.
• README.md has been updated to clearly notify users of this change and to provide manual installation guidance.
• Version bumped to 6.0.5 in kss/__init__.py.

Additional Notes

• No changes to the core functionality of kss.
• Please update documentation to clarify mecab installation requirements if necessary.

Closes: #<issue_number_if_known>

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