SERVER-15311 Never read from admin.system.new_users when looking for user documents.

The master branch no longer needs to support the 2.4-2.6 upgrade process, obviating the need for
this decision making.  Removing it will reduce the likelihood of future errors, and simplifies
storage concurrency control by removing the need for a multi-document atomic read.
SERVER-14668 DBRead should be acquiring S lock, not X

This fixes a bug introduced in commit
85e785fdce07dbb23b49220f4798f4627cf6af79.