[gh-846][gh-806] Remove $og_membership->group_type check

Unsure why it was added but it appears that it is always 'node'
so the code below never executes
[gh-846][gh-806] Reinstate $og_membership 'user' check with entity_type

b39f43c was likely a bug using group_type
Merge pull request #851 from ericras/issue-846

Closes #846 Fix $og_membership check