backup: drop nonexistent --clear-retention flag from retention helper

kopia_backup/templates/kopia-backup.j2

@@ -53,11 +53,11 @@ ignore {{ i.key | quote }} {{ i.value | map('quote') | join(' ') }}
 {% set retentions = kopia_backup.retention | default({}) | dict2items | rejectattr('value', 'none') | list %}
 {% if retentions %}
 
-# Per-source retention overrides, applied as a whole (--clear-retention runs after the keep flags within one call).
+# Per-source retention overrides, applied as a whole. A keep flag of 0 means "inherit",
+# so setting all four flags explicitly fully defines the retention for the path.
 retention() {
   local path=$1; shift
-  kopia policy set "$path" --clear-retention >/dev/null 2>&1 &&
-    kopia policy set "$path" "$@" >/dev/null 2>&1 ||
+  kopia policy set "$path" "$@" >/dev/null 2>&1 ||
     { echo "setting the retention of $path failed" >&2; failed=1; }
 }
 {% for r in retentions %}