tests: remove the unused 'root-hint' target

tests: remove the unused 'root-hint' target

* tests/Makefile.am (root-hint): Here.  The interested user can see
the reasons why some tests are skipped by looking at the messages
they display on the console; here's an excerpt:

    ...
    PASS: misc/id-groups.sh
    id-setgid.sh: skipped test: must be run as root
    SKIP: misc/id-setgid.sh
    PASS: misc/md5sum.pl
    ...
    PASS: df/total-verify.sh
    2g.sh: skipped test: very expensive: disabled by default
    SKIP: du/2g.sh
    ...

Clear enough, and more specific and precise that a generic "some tests
might need to be run as root" message.  And if that user is interested
in making those tests run anyway, he'll just take a look to the README
files to look for info.  So there's no reason to pollute the stdout
with another "hint" that is subsumed by those messages, and that might
go unnoticed anyway.

Moreover, and possibly more importantly, that hint wasn't being
displayed anyway, even before this change!  That's because the
'root-hint' target was listed as prerequisite for the 'check-recursive'
target, which however was not a dependency of the 'check' target in
'tests/Makefile.am', because that file contains no $(SUBDIRS)
definition.