[libc++] Remove dead code in legacy_debug_handler.cpp (#68155)

We removed all traces of the legacy debug mode a while back, but we
forgot to remove the actual `.cpp` file that implemented the legacy
debug handler. The file is not referenced from anywhere so this is
effectively a NFC.

Author: ldionne

libcxx/src/legacy_debug_handler.cpp

@@ -523,7 +523,6 @@ libcxx/src/include/ryu/ryu.h
 libcxx/src/include/sso_allocator.h
 libcxx/src/ios.cpp
 libcxx/src/iostream.cpp
-libcxx/src/legacy_debug_handler.cpp
 libcxx/src/locale.cpp
 libcxx/src/memory.cpp
 libcxx/src/mutex.cpp