gtkflowbox: Drop crossing/motion event handlers

gtkflowbox: Drop crossing/motion event handlers

Those basically controlled priv->active_child_active, which would
1) trigger a redraw when the pointer enters/leaves it, and 2) ensure
that press/release happen on the same child for it to be activated.

The former is not necessary, and the latter can be simplified by
just checking again the child on the coordinates given by the
::release gesture handler. This makes all enter/leave/motion_notify
event handlers unneeded.