Fix unused variables warning

UserCtx was available here in the past so we could upgrade password
hashes when authenticating. In the clustered case we found this to be
messy as multiple attempts to upgrade the same document would occur
close together. For 2.0, we elected to remove this upgrade-on-auth
path.
diff --git a/src/couch_httpd_auth.erl b/src/couch_httpd_auth.erl
index 15d3ac6..ec7ede1 100644
--- a/src/couch_httpd_auth.erl
+++ b/src/couch_httpd_auth.erl
@@ -91,7 +91,7 @@
         case AuthModule:get_user_creds(Req, User) of
             nil ->
                 throw({unauthorized, <<"Name or password is incorrect.">>});
-            {ok, UserProps, AuthCtx} ->
+            {ok, UserProps, _AuthCtx} ->
                 reject_if_totp(UserProps),
                 UserName = ?l2b(User),
                 Password = ?l2b(Pass),
@@ -298,7 +298,7 @@
     UserName = ?l2b(extract_username(Form)),
     Password = ?l2b(couch_util:get_value("password", Form, "")),
     couch_log:debug("Attempt Login: ~s",[UserName]),
-    {ok, UserProps, AuthCtx} = case AuthModule:get_user_creds(Req, UserName) of
+    {ok, UserProps, _AuthCtx} = case AuthModule:get_user_creds(Req, UserName) of
         nil -> {ok, [], nil};
         Result -> Result
     end,