The most likely outcome of reducing low-growing plants from an area is that the proportion of tortoises with the ability to extend their necks a long distance would increase.
This is because long-necked tortoises would be better adapted to reach the remaining higher vegetation for food, which would give them a survival advantage over short-necked tortoises, who would struggle to find enough food. Over time, this selection pressure would likely lead to an increase in the proportion of long-necked tortoises in the population.