Revert "meta-phosphor: disable 'uninative' for RHEL6 support"

Drop support for RHEL6 which has been unsupported by Poky
since 2.1.

This reverts commit c83cf1ce.

Tested: Built a witherspoon image
Change-Id: Idd8dda940c72a69a7a228733c34018f906ec9b8f
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>