Fix: Modify the method of obtaining the first_layer_time.

Jira: None

Change-Id: I0ef83d8f47f9d7b235b88caaa8b895367729f3e9
(cherry picked from commit 1de8d5c0d3bb6742219963bd7297286a937e0ed5)

src/libslic3r/GCode.hpp

@@ -618,8 +618,6 @@ private:
 
     std::vector<const PrintObject*> m_printed_objects;
 
-    float m_initial_layer_time;
-
     // Processor
     GCodeProcessor m_processor;