Remove unnecessary chmod commands from setup.sh.

Summary:
Commit e540d08add1161e301f563b12cc8f182a2fed0b0 removes `cp` commands,
which are followed by `chmod 644` commands (introduced in
a84e8be0c84b62ad099dd0ba38a352a9795b103d as a first fix to non-standard
umasks), in favor off `install -m0644` commands to copy configuration
files from the host to the guest. This makes `chmod 644` commands
useless.

Test Plan:
Create a brand new virtual machine starting with vagrant up and make sure
/etc/inboxapp/config.json and /etc/inboxapp/secrets.yml are world-readable.

Reviewers: dlitz

Reviewed By: dlitz

Differential Revision: https://review.inboxapp.com/D663