sign_rpm_ext.bbclass: remove prefuncs for do_package_write_rpm and do_rootfs
sign_rpm_ext.bbclass: remove prefuncs for do_package_write_rpm and
do_rootfs

in commit 393b80fa, prefuncs of do_package_write_rpm/do_rootfs
have been replace by (task)_prepend in this bbclass, so remove it.

Signed-off-by: Changqing Li <39413a79f7c860540d900674f0db3adefe89bf18@windriver.com>

jiazhang0/meta-secure-core,jiazhang0/meta-secure-core,jiazhang0/meta-secure-core,jiazhang0/meta-secure-core