TaskModel: Remove updateMapping calls that lead to bad performance

Those were missed in 48255c92293c336d77ad4072a8dd41f7a250dfd9

Task-number: QTCREATORBUG-12109
Change-Id: Ie0cf9026038dd6627593c8dd14ab52d6c1ff19cf
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>