[tabswitcher] Fix beginRemoveRows call

Summary:
To remove one row first and last should be the same index.

This hit an assert on newer Qt.

Test Plan:
I hit this booting up with no args with kate
restoring to split views each with one document.

From the calling code it seems to be trying to just remove one row.

It now starts. Hitting control+n, control_w does add/remove tabs respectively

Reviewers: #kate, dhaumann

Reviewed By: #kate, dhaumann

Subscribers: dhaumann, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D16376