Don't use NSFileManager api for path existence detection in IsNetworkDrive api.

<del>This fixes the debug issue (https://github.com/adobe/brackets/issues/5277) introduced by pull request https://github.com/adobe/brackets-shell/pull/281 and also fixes https://github.com/adobe/brackets/issues/4410 that pull request tried to fix.</del>

This simplifies the code in checking the existence of file/folder path on Mac. The original code added to fix https://github.com/adobe/brackets/issues/4410 is somehow making debug crash issue (https://github.com/adobe/brackets/issues/5277) happening sooner in a debugging session.