Include true-hitless-script folder in standalone true-hitless-image build

Removed the addition of true-hitless scripts in obmc-phosphor-image.bbappend since
they are now included in the standalone true-hitless-image build.

Change-Id: I47a59004cca9bad8760a7a966c5e6ac98a15a2ac
Signed-off-by: Baicheng Zong <baichengz@google.com>
Google-Bug-Id: 474149010
Tested: Tested building true-hitless-image and obmc-phosphor-image successfully. Both
        images include the true-hitless scripts as expected.
fusion2://NA. No fusion link for TH