update-initramfs: Remove test for direct invocation from kernel package

The direct invocation of update-initramfs in kernel package maintainer
scripts was removed in:

- Debian linux 2.6.39-1, thus in wheezy
- Ubuntu linux 4.15.0-18.19 (I think), thus in bionic
- kernel-package 12.001, thus in Debian squeeze and Ubuntu lucid

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>