src: remove unused function

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

PR-URL: nodejs#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>

src/node_file.cc

@@ -153,10 +153,6 @@ void NewFSReqWrap(const FunctionCallbackInfo<Value>& args) {
 }
 
 
-inline bool IsInt64(double x) {
-  return x == static_cast<double>(static_cast<int64_t>(x));
-}
-
 void After(uv_fs_t *req) {
   FSReqWrap* req_wrap = static_cast<FSReqWrap*>(req->data);
   CHECK_EQ(req_wrap->req(), req);