You could import your GPX file into Garmin BaseCamp (free software). Then double click the name and it will open a window with all the points listed. Click on the first point you would like to delete. It will then have an orange dot around it on the map. Then hold the shift key and click the last point you would like to delete. The section selected will now be orange on the map. Hit the delete key and it will delete them all at ounce. You can then export it back to A GPX file.