Use a Style which displays ‘end-points’ to make them easier to see…
With nothing selected, use the Move Tool on the colinear vertex and drag it to one of the other ends…
‘FredoTools’ includes a ‘Remove-Lonely-Vertices’ tool…
Useful if you have lots of them…
See this: [Plugin] FredoTools:RemoveLonelyVertices 1.2a - 20 Jul 11 • sketchUcation • 1
The download is here: SketchUp Plugins | PluginStore | SketchUcation
You also need to install the latest LibFredo from here: SketchUp Plugins | PluginStore | SketchUcation