src: remove unused IsInt64() function

[cjihrig]

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

src

Description of change

The function IsInt64() in node_file.cc is no longer used, and was creating a warning at compile time. This commit removes the unused function.

[cjihrig]

CI: https://ci.nodejs.org/job/node-test-pull-request/3388/

[mscdex]

LGTM

[cjihrig]

Note: If #7846 lands, then this function becomes used again.

[ChALkeR]

Let's block this until there is a resolution on #7846.

[jasnell]

Given that the decision has been made to do the revert in #7846, this one likely can't land just yet. @cjihrig -- do you want to keep it open and blocked or go ahead and close for now?

[cjihrig]

I'm fine with leaving it open until there is a proper resolution one way or the other.

[cjihrig]

This function is used again, as of c5a18e7. Closing this.