sequoia-nixbld-user-migration: disable trace mode

Was hoping to leave this enabled for a little while as core community
members test this script out, but Apple's aggressive release timeline
for macOS 15 Sequoia has caught us off-guard here.

It's probably not ideal for a general audience if the script spews all
of this output--and people can still force bash to run in trace mode
if we really need to debug a problem.

scripts/sequoia-nixbld-user-migration.sh

@@ -1,7 +1,5 @@
 #!/usr/bin/env bash
 
-set -x
-
 ((NEW_NIX_FIRST_BUILD_UID=351))
 ((TEMP_NIX_FIRST_BUILD_UID=31000))