Fix build due to golang/oauth2 incompatibility

golang/oauth2@0ae3d4edc99f765dc1513a9605aa96e82877a20a changes type of
Token.Extra. So our ExtraData() broken, but it doesn't seem to be used,
just remove unused function to fix the build.
Merge pull request #36 from sorah/oauth2-compat

Fix build due to golang/oauth2 incompatibility