Manual:$wgGroupsRemoveFromSelf

Details
$wgGroupsRemoveFromSelf allow users to remove their own user rights.

 The syntax since 1.14 is: 

 The syntax of MW 1.12 - 1.13 is: 

Changes in 1.14.0
In r39368, the way this variable works was changed. Before, it allowed every user to remove groups from themselves, now it can be filtered on a per-group basis. The new syntax is   which allows any user in the group "group1" to remove the groups "group2" and "group3" from themselves. The old syntax will still work, allowing every user to remove groups from themselves, but it is recommended that you use  instead.

You may use   to allow users in that group to remove every group from themselves.