doc: remove comment about using thread_local

Followup to bitcoin/bitcoin#30095 (comment).

Author: fanquake

src/test/util/random.h

@@ -14,9 +14,8 @@
 /**
  * This global and the helpers that use it are not thread-safe.
  *
- * If thread-safety is needed, the global could be made thread_local (given
- * that thread_local is supported on all architectures we support) or a
- * per-thread instance could be used in the multi-threaded test.
+ * If thread-safety is needed, a per-thread instance could be
+ * used in the multi-threaded test.
  */
 extern FastRandomContext g_insecure_rand_ctx;