config/zfs-build.m4: never define _initramfs in RPM_DEFINE_UTIL

The zfs-initramfs package has never worked as no RPM-based distribution
uses initramfs-tools, which is listed as a dependency of zfs-initramfs.

This would not ordinarily be a problem, as it is only enabled when
/usr/share/initramfs-tools is present, which should not normally be the
case on RPM-based distributions. However, other packages may install
unused files there even if initramfs-tools is not used, so remove this
auto-detection for the rpm-utils target.

This does not fully remove the logic for the zfs-initramfs package. This
splits it out into a separate rpm-utils-initramfs target so that the
Debian builds can still use it.

Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Harald van Dijk <harald@gigawatt.nl>
Closes #10898