Get rid of persistent-help plugin (#757).

* helm-source.el (helm-source): Don't plug persistent-help.
(helm-source-sync,
helm-source-in-buffer): same.
(helm-source--header-line,
helm-source--persistent-help-string): new.
(helm--setup-source): Use them.
* helm-source.el (helm-source): Initialize header-line to nil.