how do I remove unwanted shadows from photos

You can lighten all shadows using the "Shadows & Highlights" Adjustment controls.

But photos does not have brushes to lighten individual shadows, like Aperture had.

if the Shadows&Highlight sliders are not visible, open a photo in "Edit" mode, click the Adjustments button, then click the disclosure arrow to the right of "light" to reveal the advanced controls:

If you need tor remove an individual shadow, try the "retouch" brush, but ou will probably have more success with an external editor.