(22329) Only delete puppet files in the preflight

Previously, the OSX preflight removed the contents of the 'hiera' directory
which also happens to be where the puppet.pkg places the contents of
hiera_puppet. The fix is simple – from the hiera dir, only remove the file
that exist in the 'puppet' project, not those in puppet. This commit
accomplishes this by only deleting files, not the directories. It's a bit
crufty, but its better than clobbering the wrong files. With the puppet
directory, we can still just remove it.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>