README: Deleting signed file, "make download os" command can generate new signed file.
README: Deleting signed file, "make download os" command can generate
new signed file.

Change-Id: Ica5a45b7e064fef2c40012a37c64d2b6806327e0
Signed-off-by: Youngdae Oh <047e3df7c3b25e15d5695c6d5f9d0a5908e83e0d@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT