remove unnecessary symbol filelock in IndexFile.java

[luxward]

What is the purpose of the change

trivial cleanups: remove unnecessary symbol filelock in IndexFile.java

Brief changelog

remove unnecessary symbol filelock in IndexFile.java, only remain annotation for intention of this symbol

Verifying this change

simple code review

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[coveralls]

Coverage decreased (-0.1%) to 49.408% when pulling 1db828f on luxward:remove_symbol into 430c62e on apache:develop.

[luxward]

@zhouxinyu please help me review this code. However, result of code checking is straight