Revert "Git: Undo changes on pull with rebase when user chooses not to stash"

The dialog text doesn't state that the changes will be
discarded. This is likely to be an unexpected behavior.

Will improve the dialog on master

This reverts commit cf41cd5e2a9b1927fd533789a30cfd80a4e0791e

Task-number: QTCREATORBUG-7031
Change-Id: I52eaf3cdef1b10b261ee9d2e9e14aa98e6e32bdd
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>