SERVER-9518 Don't override internal security user if there's a user doc with the same name
Revert "SERVER-9518 Fix getAllV1PrivilegeDocsForDB in mongod by using the proper namespace for system.users"

This reverts commit c5afca5be18c26faeb7a8c0ca6ad8281d7dc376c.
SERVER-9518 Remove UserName argument from grantInternalAuthorization
SERVER-6246 Add removeUsers command
SERVER-6246 Restrict direct removes from system.users, add a shell helper to use the removeUser command, and update tests to use the helper