tools/tests.sh: Remove check for commits touching external/

Since git subtree seems less fussy these days, remove the check that
prevented touching external/, since it does not mix well with GitHub's
"rebase and merge" feature.

"rebase and merge" leads to a loss of merge commits, which makes the
commit being detected as "illegal" by the current tests.sh. Since git
subtree nowadays seems to be able to recover from that situation without
complaining, remove the check.