src: remove unused function

The function IsInt64() in src/node_file.cc is no longer used.
This commit removes it.

PR-URL: https://github.com/nodejs/node/pull/17671


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Jon Moss <me@jonathanmoss.me>