Cleanup: remove unsupported retcheck.c, rct.c, loadtoconst.c, etc.

retcheck.c's check_debug and other clean calls are stale.

There is no reproducer.

By inspection, the debug_check code and some other clean call functions seem stale. Some of the functions take in parameters (e.g. errno, registers) that are not declared in the clean call.

The code should get further inspected and cleaned up.