[9.18] Don't undef <unit>_TRACE, instead add comment how to enable it

In units that support detailed reference tracing via ISC_REFCOUNT macros, we were doing:

/* Define to 1 for detailed reference tracing */
#undef <unit>_TRACE

This would prevent using -D_TRACE=1 in the CFLAGS.

Convert the above mentioned snippet with just a comment how to enable the detailed reference tracing:

/* Add -D<unit>_TRACE=1 to CFLAGS for detailed reference tracing */

(cherry picked from commit 6afa9615)

Backport of !8381 (merged)