py: Cleanup use of global DEBUG preprocessor definition

The standard preprocessor definition to differentiate debug and non-debug
builds is NDEBUG, not DEBUG, so don't rely on the latter:
- just delete the use of it in objint_longlong.c as it has been stale code
  for years anyway (since commit [c4029e50]): SUFFIX isn't used anywhere.
- replace DEBUG with MICROPY_DEBUG_NLR in nlr.h: it is rarely used anymore
  so can be off by default