build_image: Don't add root_hash.txt to the final tarball

Adding the `root_hash.txt` to the final tarball doesn't bring any
benefit to the project, as the file dependency is for building the
shim-v2 and passing the correct measurement for the kernel command line.

It's important to mention that when building shim-v2, it doesn't look
for the file in `/opt/confidential-containers/share/kata-containers`,
bur rather in the `${repo_root_dir}/tools/osbuilder/`, as shown here:
ac3683e26e/tools/packaging/kata-deploy/local-build/kata-deploy-binaries.sh (L228-L232)

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

tools/packaging/guest-image/build_image.sh

@@ -73,9 +73,6 @@ build_image() {
 		IMG_OS_VERSION="${img_os_version}" \
 		ROOTFS_BUILD_DEST="${builddir}/rootfs-image"
 	mv -f "kata-containers.img" "${install_dir}/${image_name}"
-	if [ -e "root_hash.txt" ]; then
-	    cp root_hash.txt "${install_dir}/"
-	fi
 	(
 		cd "${install_dir}"
 		ln -sf "${image_name}" "${final_image_name}${image_initrd_extension}"