Fix a crash when cancelling Transform Tool action

The crash happens only on systems that have asserts enabled, that
is, don't have NDEBUG defined.

Binding uninitialized '*m_savedTransformArgs' to a cont-reference
generated valid c++ code (a reference initialized with nullptr),
and given that finishStrokeImpl() didn't try to use/dereference
this reference, the code worked fine and didn't crash on systems
without asserts.

But on systems with asserts enabled (non NDEBUG), boost::optional
triggered a sanity check assert for dereferencing nullptr and crashed
the application.

The patch removes entire code for resetting m_savedTransformArgs in
cancelStrokeCallback(). It was actually an artifact of some initial
refactoring.

Thanks Fredrik Hansson for pointing out the problem!
https://invent.kde.org/kde/krita/merge_requests/197

BUG:414672