Revert "Fix dx find executions in public projects" - There's a simpler way.

This reverts commit b15ac9fb7b9dab50da6f264889bc530ac18d0e51.
This is a simplified version of b15ac9fb7b9dab50da6f264889bc530ac18d0e51 (see comment in the commit).
Fix mmap check on Mac OS.

On Mac OS the system reports a nonzero size when you stat stdin. We
assumed this would always be 0 for files that cannot be mmap'd. This
caused "dx upload -" to fail on Macs.

Fix this check by replacing it with checks on the stat mode.