RFR: 8355617: Remove historical debug_only macro in favor of DEBUG_ONLY

[Aleksey Shipilev]

I noticed in `macros.hpp`:


#ifdef ASSERT
#define DEBUG_ONLY(code) code
#define NOT_DEBUG(code)
#define NOT_DEBUG_RETURN /*next token must be ;*/
// Historical.
#define debug_only(code) code
#else // ASSERT


There are 350+ instances of `debug_only`, and 1600+ instances of `DEBUG_ONLY`. We can cleanup `debug_only` and rewrite all uses to "DEBUG_ONLY". 

I think we can do this in one shot. This was a mechanical rename. I have looked through the `grep -R debug_only src/ | nl`, and there are no hits anymore.

Additional testing:
 - [ ] GHA

-------------

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/24879/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24879&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355617
  Stats: 371 lines in 119 files changed: 0 ins; 3 del; 368 mod
  Patch: https://git.openjdk.org/jdk/pull/24879.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24879/head:pull/24879

PR: https://git.openjdk.org/jdk/pull/24879