webmin: remove init script for gentoo

The configure.initd.gentoo script is used for gentoo, it is invalid for oe. So
remove it to solve the following warning:

webmin-1.700: webmin-module-ajaxterm requires /sbin/runscript, but no providers
in its RDEPENDS [file-rdeps]

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>