Remove legacy codes.
Use SessionStore.jsm if it's available.
Remove the needless comment.

By mxr, this comment doesn't need:
http://mxr.mozilla.org/mozilla-central/search?string=SessionStore.jsm&find=&findi=&filter=^[^\0]*%24&hitlimit=&tree=mozilla-central
Merge pull request #57 from saneyuki/legacy

Remove legacy codes