tracing: correctly scope utxocache:flush tracepoint

Previously, the `utxocache:flush` tracepoint was in the wrong scope and
reached every time `CChainState::FlushStateToDisk` was called, even when
there was no flushing of the cache. The tracepoint is now properly scoped
and will be reached during a full flush.

Inside the scope, the `fDoFullFlush` value will always be `true`, so it
doesn't need to be logged separately. Hence, it's dropped from the
tracepoint arguments.

src/validation.cpp

@@ -2324,14 +2324,13 @@ bool CChainState::FlushStateToDisk(
                 return AbortNode(state, "Failed to write to coin database");
             nLastFlush = nNow;
             full_flush_completed = true;
+            TRACE5(utxocache, flush,
+                   (int64_t)(GetTimeMicros() - nNow.count()), // in microseconds (µs)
+                   (u_int32_t)mode,
+                   (u_int64_t)coins_count,
+                   (u_int64_t)coins_mem_usage,
+                   (bool)fFlushForPrune);
         }
-        TRACE6(utxocache, flush,
-               (int64_t)(GetTimeMicros() - nNow.count()), // in microseconds (µs)
-               (u_int32_t)mode,
-               (u_int64_t)coins_count,
-               (u_int64_t)coins_mem_usage,
-               (bool)fFlushForPrune,
-               (bool)fDoFullFlush);
     }
     if (full_flush_completed) {
         // Update best block in wallet (so we can detect restored wallets).