Drop support for GDAL 1.x

Rationale

OTB currently support both GDAL 1.X (X>10) and GDAL 2.X, this was useful when the second version came out, because a lot of users were still using GDAL 1, but this is hard to maintain, as OTB has to be compatible with both API : it increases the code volume and make it harder to test. Now that GDAL 2 has been around for some time, it is time to drop support for GDAL 1.X