Do not unconditionally install the core snap

Seeing any snap via snap_preseed will evaluate the base for each snap
and seed the appropriate base.  There should be no reason to explicitly
seed the 'core' snap and with snaps moving to 'core18' this will add
'core' without need.

live-build/functions

@@ -620,12 +620,6 @@ snap_prepare() {
     local CUSTOM_BRAND_MODEL=${2:-generic:generic-classic}
 
     snap_prepare_assertions "$CHROOT_ROOT" "$CUSTOM_BRAND_MODEL"
-
-    # ubuntu-cpc:minimized has its own special snap handling
-    if [ "$PROJECT:${SUBPROJECT:-}" != ubuntu-cpc:minimized ]; then
-        # Download the core snap
-        _snap_preseed "$CHROOT_ROOT" core stable
-    fi
 }
 
 snap_preseed() {