Try to reduce build time and do not build initramfs twice.

Lets see whether this might break in any situation (thinking of
certain combinations of softupdate, renames,... during release
management) but building the initramfs takes just too long, so let's
try to reduce the build time via building it only from within main
grml-live script and no longer execute update-initramfs in the
/etc/grml/fai/config/scripts/GRMLBASE/10-build-initramfs script.