Extraneous `clearFeedbackHandlers()` in 02a

[rLinks234]

hebi-cpp-examples/basic/02a_feedback_foreground.cpp

Line 39 in e659ef3

group->clearFeedbackHandlers();

This is an example which erroneously infers to the user that they must clear feedback handlers, even when none were set.

Even further, feedback handlers are automatically destructed when the group is destructed, so we need to remove this extraneous call.

[iamtesch]

This is extraneous, and we should remove, but it doesn't actually hurt anything to have the extra call. It just means a list is cleared. There is no double free or anything that occurs.