Revert "use existing salt during sha1->pbkdf2 upgrade to avoid conflicts"

This reverts commit 569338222a9ee38c35d2270b2a2a405be26a0828.

I think I'm wrong about conflicts, we update the document before
we write (in fabric).
diff --git a/src/couch_users_db.erl b/src/couch_users_db.erl
index 74ff090..3b76862 100644
--- a/src/couch_users_db.erl
+++ b/src/couch_users_db.erl
@@ -64,7 +64,7 @@
         Doc;
     ClearPassword ->
         Iterations = list_to_integer(config:get("couch_httpd_auth", "iterations", "1000")),
-        Salt = couch_util:get_value(?SALT, Body, couch_uuids:random()),
+        Salt = couch_uuids:random(),
         DerivedKey = couch_passwords:pbkdf2(ClearPassword, Salt, Iterations),
         Body0 = [{?PASSWORD_SCHEME, ?PBKDF2}, {?ITERATIONS, Iterations}|Body],
         Body1 = ?replace(Body0, ?DERIVED_KEY, DerivedKey),