rpm-postinsts: don't create broken postinst script

rpm-postinsts: don't create broken postinst script

Not only was the variable reference in this line broken, but it wasn't
going to work anyway - we install the script directly into /etc/rcS.d
and not into /etc/init.d, so the code in update-rc.d.bbclass couldn't
find anything there. This resulted in a postinstall script for
rpm-postinsts being created in /etc/rpm-postinsts which can't work when
the root filesystem is read-only. To simplify things just remove the use
of update-rc.d.bbclass since we don't really need the added complexity
here.

Fixes [YOCTO #4222].

(From OE-Core rev: d196d08acafe599c16a7ac8e04121039b1216ba6)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>