Remove `node_under_mouse_tweak` and replace it with
`node_under_mouse_select`.
The special handling for tweaking reroutes wasn't working correctly:
1. It didn't check at the correct reroute node location.
2. The (de-)selection behavior negated possible benefits anyway.
In practice the user-facing behavior is not affected by this.
Pull Request: https://projects.blender.org/blender/blender/pulls/130222
cff7130fc2