drpcdebug: remove dead package

The drpcdebug package provided build-tag-gated debug logging that was
activated only when building with `-tags=debug`. Investigation shows it
was effectively dead code:

- No CI configuration or Makefile ever enabled the debug build tag
- No tests validated the debug-enabled code path
- CockroachDB (the primary consumer) never used it
- Last substantive change was in June 2021
- Only 4 callsites existed, all now replaced by Logger.Debugf

All functionality has been migrated to the Logger interface introduced
in the previous commits. The drpcdebug package is no longer imported
anywhere and can be safely deleted.

Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>

Author: cthumuluru-crdb

README.md

@@ -210,6 +210,5 @@ DRPC is proud to get as much done in as few lines of code as possible. It's the
 | storj.io/drpc/drpcctx                | 41       |
 | storj.io/drpc/internal/drpcopts      | 30       |
 | storj.io/drpc/drpcstats              | 25       |
-| storj.io/drpc/drpcdebug              | 22       |
 | storj.io/drpc/drpcenc                | 15       |
-| **Total**                            | **3611** |
+| **Total**                            | **3589** |