tar: minor refactoring of populateTree and flatten
tar: minor refactoring of populateTree and flatten

Cleaned up some dead code, and made populateTree not
take in a *tar.Streamer argument, as the ts argument was
only used to set an error. The function now returns
an error (if any). Also made flatten not have to take in
a *tar.Streamer argument as well.

Signed-off-by: Stephen Chung <dfcd4e1043ff9779fc85be4e40636ec800ab4d59@redhat.com>

vbatts/go-mtree,vbatts/go-mtree