Fix lint issues in qlib/utils/ (W1309, W0612)

[Bortlesboat]

Summary

• Remove unnecessary f prefix from 16 string literals in qlib/utils/ that contain no interpolated variables (pylint W1309: f-string-without-interpolation)
• Prefix unused variable lft_etd with underscore in qlib/utils/data.py to indicate it is intentionally unused (pylint W0612: unused-variable)

Contributes to #1007

Details

Files changed:

• qlib/utils/index_data.py — 7 f-string fixes
• qlib/utils/objm.py — 7 f-string fixes
• qlib/utils/mod.py — 1 f-string fix
• qlib/utils/time.py — 1 f-string fix
• qlib/utils/data.py — 1 unused variable fix (lft_etd → _lft_etd)

All changes are safe cosmetic/lint fixes with no behavioral impact. The existing flake8 and pylint CI checks continue to pass, and the previously-disabled W1309 and W0612 rules now pass cleanly for qlib/utils/.

Test plan

• Verified flake8 passes with project config (make flake8 equivalent)
• Verified pylint passes with project config (make pylint equivalent)
• Verified W1309 (f-string-without-interpolation) reports 0 issues in qlib/utils/
• Verified W0612 (unused-variable) reports 0 issues in qlib/utils/
• All changes are purely cosmetic — no logic or behavior changes

🤖 Generated with Claude Code

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