topic: use registrar.revsetpredicate to register revset predicate functions
Now, using registrar.revsetpredicate of Mercurial directly in topic
extension should be safe enough. because it has been available since
Mercurial 3.8, and minimum Mercurial version for topic extension is
4.0.
This patch also removes modsetup() in topic/revset.py, because it is
useless.
BTW, this patch fixes an issue of "hg help revsets" output that custom
revset predicates are not displayed as same as others, because they
are not "inline literal" in reST syntax (``NAME``).