To edit a line feature and delete vertices (but not delete the entire feature):
Step 1: Edit the line
Navigate to the record containing the feature. In this case, it's within a Site Visit. Click on the three dots to the left of the feature and click 'Edit Layer'.
Step 2: Click on the vertices that you want to remove:
Click the thumbs up to save your edits, or the thumbs down to cancel them:
You only need to use the Trash Can / Delete Item feature if you need to delete an entire feature completely.
If you have an exceptionally long line to delete (like if you forgot to turn off tracking before getting in the car) you may want to export the line, edit it in another platform, and then reimport it. This video shows you how to do so with Google Earth.