fix double emitting of value_changed event in _on_value_change function

[RichardFrangenberg]

Removed a line which caused that value_changed was emitted twice when a vector widget value was changed.

Also fixed a problem that the _value variable of a vector wasn't replaced with a new list when a _NumberValueEdit was changed directly. This caused that the NodeGraph class didn't emit the property_changed signal when changing the number fields of a PropColorPickerRGB widget more than once.
This is because this if condition would be false in these cases:

NodeGraphQt/NodeGraphQt/base/graph.py

Line 257 in 0eb0262

if node.get_property(prop_name) != prop_value:

I hope it makes sense, but I can provide more details if needed.

[jchanvfx]

Awesome thanks for catching that 😃