Remove obsolete legacy headers

[skilledwolf]

Summary

• remove the four opt-in deprecated compatibility headers under legacy/alps
• remove the unused ALPS_INSTALL_DEPRECATED CMake option and install rule
• stop the line-count helper from scanning the removed directory

These headers were deprecated in 2010, are not used within ALPS, are disabled by default, and have no test coverage. The alps/vectorio.h shim was also ineffective because it included itself.

Migration

• alps/copyright.h → alps/utility/copyright.hpp
• alps/osiris/os.h → alps/utility/os.hpp
• alps/vectorio.h → alps/utility/vectorio.hpp
• alps/math.hpp → the required headers under alps/numeric/

Checks

• fresh minimal CMake configuration
• script/linecount.sh
• no remaining references to legacy/alps or ALPS_INSTALL_DEPRECATED
• git diff --check