Don't assign twice the same key to the action New Tab

Summary:
QKeySequence::AddTab = Qt::CTRL + Qt::Key_T.
Leaving only the first avoids the warning window "There are two actions (New Tab, New Tab) that want to use the same shortcut (Ctrl+T)"

BUG: 398324

Test Plan:
su - test
unset KDE_FULL_SESSION
dolphin
After: No more the warning window.

Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D15371