prepare: Never sync Portage repos

To minimize unexpected changes between builds, I'm going to schedule a
separate task to sync the Portage repositories.  This way, we know that
two runs in a row from the same source will have the same packages,
unless we have specifically updated Portage.

prepare.sh

@@ -2,13 +2,6 @@
 
 . "${CONFIGDIR:=${PWD}}"/config
 
-if [ ! -f /var/db/repos/gentoo/metadata/timestamp ]; then
-    emerge-webrsync
-fi
-if [ "$(find /var/db/repos/gentoo/metadata -newermt '-24 hours' | wc -l)" -eq 0 ]; then
-    emaint sync
-fi
-
 mkdir -p /etc/portage/package.use
 mkdir -p /etc/portage/make.conf
 echo 'virtual/libudev systemd' >> /etc/portage/package.use/systemd