This reverts commit b3c3a183.

Remove unused commented code.
Rename OTB_xxx from otbGeometryMetadata to simply xxx.
Optimize constructors for otbGeometryMetadata::GCP.
Use reference to MetadataSupplierInterface instead of pointer in
ImageMetadataInterfaceBase::Fetch.
Use keyword 'using' in place of 'typedef' in GDALImageMetadataInterface.

The class XMLMetadataSupplier belongs to a public hierarchy,
it should not be possible to copy it.