Remove redundant HGTD namespaces
This MR touches many files, but the changes can be summarised as the removal of 3 namespaces for HGTD which are all redundant due to HGTD already appearing in the names in question:
HGTDet was being used for HGTD tracking geometry,
HGTDGeo for the HGTD detector factory, and
HGTD for the HGTD clusters. Equivalent removal of namespaces also to follow soon in 21.9.