Unused variable cleanup (#13917)

Marlin/src/gcode/feature/pause/M701_M702.cpp

 
     const int8_t target_extruder = active_extruder;
   #else
-    const float unload_length_multiplier = 1.0;
     const int8_t target_extruder = get_target_extruder_from_command();
     if (target_extruder < 0) return;
   #endif

Marlin/src/sd/cardreader.cpp

   SdFile *sub = &newDir1, *startDir;
 
   const char *dirname_start = path;
-  char echo_fn[105];
-  
   if (path[0] == '/') { 
     curDir = &root;
     workDirDepth = 0;