BUILD: Remove detection of libcheck
    
    The unit test framework check has not been used in freeipa for long time
    (if ever) but there was still conditional check for this framework.
    It just produced confusing warning:
        Without the 'CHECK' library, you will be unable
        to run all tests in the 'make check' suite
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>