Fix small memory leak in get_dbname_oid_list_from_mfile().

Fix small memory leak in get_dbname_oid_list_from_mfile().

Coverity complained that this function leaked the dumpdirpath string,
which it did.  But we don't need to make a copy at all, because
there's not really any point in trimming trailing slashes from the
directory name here.  If that were needed, the initial
file_exists_in_directory() test would have failed, since it doesn't
bother with that (and neither does anyplace else in this file).
Moreover, if we did want that, reimplementing canonicalize_path()
poorly is not the way to proceed.  Arguably, all of this code should
be reexamined with an eye to using src/port/path.c's facilities, but
for today I'll settle for getting rid of the memory leak.