Clean usage of QmitkInteractionSchemeToolBar and InteractionSchemeSwitcher

Summary:
Remove unnecessary function, member and header

Do not explicitly set the visibility of the tool bar

CreateQtPartControl already calls OnPreferencesChanged, which in
turn calls OnInteractionSchemeChanged with the retrieved scheme preference.
Here the visibility of the tool bar is correctly set.

Do not set the interaction scheme

When defining a new interaction event handler there is no need to
set a default interaction scheme.
A developer using this class needs to make sure that a valid
interaction event is loaded elsewhere.

Remove member variable and create new variable on demand

Member variable is rarely used - only if the interaction scheme is switched.
Creating a new interaction scheme switcher on demand is enough for the
tool bar to work.

Test Plan:
manual tool bar testing

• StdMultiWidget / MxNMultiWidget
• MITK interaction scheme / PACS interaction scheme

Reviewers: s434n, O1 MITK Reviewer Group I, O3 MITK Reviewer Group II, kislinsk

Reviewed By: s434n, O1 MITK Reviewer Group I, O3 MITK Reviewer Group II, kislinsk

Subscribers: kalali, s434n

Maniphest Tasks: T24358

Differential Revision: https://phabricator.mitk.org/D549