Fix split_live_squashfs_layers crash under POSIX sh (dash)

trap RETURN is a bash extension not supported by /bin/sh on Debian.
With set -e active the unsupported trap call exited the build immediately
after lb build, before bootloader sync and ISO copy steps ran.

Remove both trap RETURN lines — explicit rm -rf at the end of the
function is sufficient for cleanup on the happy path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/build.sh

@@ -986,7 +986,6 @@ split_live_squashfs_layers() {
     tmp_root="$(mktemp -d)"
     tmp_usr="$(mktemp -d)"
     tmp_fw="$(mktemp -d)"
-    trap 'rm -rf "$tmp_root" "$tmp_usr" "$tmp_fw"' RETURN
 
     echo "=== splitting live squashfs into smaller layers ==="
     unsquashfs -d "$tmp_root/root" "$base_sq" >/dev/null
@@ -1013,7 +1012,6 @@ split_live_squashfs_layers() {
     echo "=== live squashfs layers ==="
     find "$live_dir" -maxdepth 1 -type f -name '*.squashfs' -exec du -sh {} \; | sort
     rm -rf "$tmp_root" "$tmp_usr" "$tmp_fw"
-    trap - RETURN
 }
 
 recover_iso_memtest() {