Inline manage_inactivity, tweak autoreport_paused

Marlin/src/core/utility.h

@@ -27,14 +27,11 @@
 // Delay that ensures heaters and watchdog are kept alive
 void safe_delay(millis_t ms);
 
-// A delay to provide brittle hosts time to receive bytes
-inline void serial_delay(const millis_t ms) {
-  #if ENABLED(SERIAL_OVERRUN_PROTECTION)
-    safe_delay(ms);
-  #else
-    UNUSED(ms);
-  #endif
-}
+#if ENABLED(SERIAL_OVERRUN_PROTECTION)
+  void serial_delay(const millis_t ms);
+#else
+  inline void serial_delay(const millis_t) {}
+#endif
 
 #if GRID_MAX_POINTS_X && GRID_MAX_POINTS_Y