Revert "SERVER-15515 user updates in mixed version replSet"

This reverts commit 9e58312f0363b8164b59dc2541b733d7417a9986.
Revert "SERVER-15500 New test for system.user operations"

This reverts commit 0a98d654a7054d252dd921d27a5649f7177c8723.
BUMP 2.6.5
post 2.6.5
Merge branch 'v2.6' of github.com:mongodb/mongo into v2.6
SERVER-15500: System users modifications